Vegetarian Spaghetti Squash Casserole

Why You’ll Love This Recipe

I love this casserole because it turns simple vegetables into a warm and satisfying dinner. The spaghetti squash develops a tender, noodle-like texture, while the tomato sauce and Italian herbs give every bite plenty of savory flavor.

I also appreciate how easy it is to customize. I can add whatever vegetables I have available, adjust the amount of cheese, or add beans for extra protein. Since I can prepare several components ahead of time, it also works well when I want to make dinner preparation easier later in the day.

Ingredients

(Tip: You’ll find the full list of ingredients and measurements in the recipe card below.)

• Spaghetti squash
• Olive oil
• Yellow onion
• Garlic
• Red bell pepper
• Zucchini
• Baby spinach
• Marinara sauce
• Italian seasoning
• Dried oregano
• Salt
• Black pepper
• Ricotta cheese
• Shredded mozzarella cheese
• Grated Parmesan cheese
• Fresh basil or parsley for serving
• Red pepper flakes, optional

Directions

1. I preheat my oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. I carefully cut the spaghetti squash in half lengthwise and scoop out the seeds. I drizzle the cut sides with a little olive oil and season them with salt and black pepper.
3. I place the squash cut-side down on the baking sheet and roast it for about 35–45 minutes, or until it is tender enough to easily pierce with a fork.
4. While the squash roasts, I heat a little olive oil in a large skillet over medium heat. I add the chopped onion and bell pepper and cook until they begin to soften.
5. I add the zucchini and garlic and continue cooking for a few minutes. Then I stir in the spinach and cook just until it wilts.
6. I add the marinara sauce, Italian seasoning, oregano, salt, black pepper, and optional red pepper flakes. I let the mixture simmer briefly so the flavors come together.
7. Once the squash is cool enough for me to handle, I use a fork to scrape the flesh into spaghetti-like strands.
8. I combine the spaghetti squash strands with the vegetable and tomato mixture. I gently stir in the ricotta and some of the mozzarella and Parmesan.
9. I transfer everything to a lightly greased casserole dish and spread it into an even layer. I sprinkle the remaining mozzarella and Parmesan over the top.
10. I bake the casserole for about 20–25 minutes, until it is hot and bubbling and the cheese has melted. For a more golden top, I broil it briefly at the end while watching it closely.
11. I let the casserole rest for about 5–10 minutes before serving. I finish it with fresh basil or parsley.

Servings and Timing

I make this recipe as approximately 6 servings, making it a convenient choice for a family dinner or meal prep.

• Prep time: 15 minutes
• Squash roasting time: 35–45 minutes
• Casserole baking time: 20–25 minutes
• Total time: approximately 1 hour 15 minutes
• Servings: 6

Variations

For extra protein, I like adding drained chickpeas, cannellini beans, or cooked lentils to the tomato and vegetable mixture. They make the casserole more substantial without adding meat.

When I want even more vegetables, I add mushrooms, broccoli, kale, eggplant, or chopped artichoke hearts. I make sure to cook vegetables that release a lot of moisture before adding them to the casserole so the finished dish doesn’t become watery.

For a different cheese combination, I sometimes replace the ricotta with cottage cheese or use provolone along with the mozzarella. When I want a little more heat, I increase the red pepper flakes or use a spicy marinara sauce.

For a vegan variation, I use dairy-free ricotta, mozzarella, and Parmesan alternatives. I also check the marinara sauce ingredients to make sure they fit my preferred diet.

Storage/Reheating

I let leftover casserole cool before transferring it to an airtight container. I store it in the refrigerator for up to 3–4 days.

When reheating an individual serving, I use the microwave until the center is thoroughly heated. For a larger amount, I place the casserole in an oven-safe dish, cover it loosely with foil, and warm it at 350°F (175°C) until heated through.

I can also freeze portions for longer storage, although the spaghetti squash may become slightly softer after thawing. I prefer freezing individual portions in airtight freezer-safe containers and thawing them overnight in the refrigerator before reheating.

FAQs

How do I keep spaghetti squash casserole from becoming watery?

I avoid overcooking the spaghetti squash and make sure vegetables such as zucchini, mushrooms, and spinach release their excess moisture in the skillet before I assemble the casserole. If my cooked squash seems particularly wet, I also let the strands drain briefly before mixing them with the sauce.

Can I prepare Vegetarian Spaghetti Squash Casserole ahead of time?

Yes. I can roast the squash, prepare the vegetable sauce, and assemble the casserole ahead of time. I cover and refrigerate it, then bake it when I am ready to serve. I may need to add a few extra minutes to the baking time when it goes into the oven cold.

What can I serve with spaghetti squash casserole?

I usually serve it with a simple green salad, roasted vegetables, or garlic bread. Since the casserole already contains plenty of vegetables and cheese, I find that a simple side dish is usually enough to complete the meal.

Can I make this spaghetti squash casserole vegan?

Yes. I replace the ricotta, mozzarella, and Parmesan with plant-based alternatives. I can also add beans or lentils when I want to make the vegan version more filling.

How do I know when spaghetti squash is cooked?

I know my spaghetti squash is ready when I can easily pierce the flesh with a fork and pull it into tender strands. I try not to roast it until extremely soft because slightly firm strands hold their texture better once the casserole is baked.

Conclusion

This Vegetarian Spaghetti Squash Casserole is a comforting recipe I love making when I want a vegetable-packed dinner that still feels warm and satisfying. The combination of roasted spaghetti squash, savory tomato sauce, vegetables, herbs, and melted cheese creates plenty of flavor without making the preparation complicated.

I especially like how adaptable the casserole is. I can change the vegetables, add plant-based protein, experiment with different cheeses, or prepare it ahead of time. Whether I make it for a weeknight meal or save portions for later, it’s an easy vegetarian casserole I’m happy to return to again and again.

Vegetarian Spaghetti Squash Casserole

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A hearty, cheesy vegetarian casserole made with tender spaghetti squash, colorful vegetables, savory marinara, Italian herbs, and melted cheese. It delivers all the comfort of a baked pasta casserole with plenty of vegetables in every serving.

• Author: Laura
• Prep Time: 15 minutes
• Cook Time: 60 minutes
• Total Time: 1 hour 15 minutes
• Yield: 6 servings
• Category: Main Course
• Method: Baking
• Cuisine: Italian-Inspired
• Diet: Vegetarian

Ingredients

• 1 large spaghetti squash
• 2 tablespoons olive oil, divided
• 1 medium yellow onion, chopped
• 3 cloves garlic, minced
• 1 red bell pepper, chopped
• 1 medium zucchini, chopped
• 2 cups baby spinach
• 3 cups marinara sauce
• 1 teaspoon Italian seasoning
• 1/2 teaspoon dried oregano
• 1/2 teaspoon salt, plus more for seasoning the squash
• 1/4 teaspoon black pepper, plus more for seasoning the squash
• 1 cup ricotta cheese
• 2 cups shredded mozzarella cheese, divided
• 1/2 cup grated Parmesan cheese, divided
• Fresh basil or parsley, for serving
• Red pepper flakes, optional

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Instructions

1. Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. Carefully cut the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the cut sides with some of the olive oil and season with salt and black pepper.
3. Place the squash cut-side down on the prepared baking sheet and roast for 35–45 minutes, or until the flesh can easily be pierced with a fork and pulled into strands.
4. While the squash roasts, heat the remaining olive oil in a large skillet over medium heat. Add the onion and red bell pepper and cook until they begin to soften.
5. Add the zucchini and garlic and cook for a few more minutes. Stir in the spinach and cook just until wilted.
6. Add the marinara sauce, Italian seasoning, oregano, salt, black pepper, and optional red pepper flakes. Simmer briefly to combine the flavors.
7. When the roasted squash is cool enough to handle, scrape the flesh with a fork to create spaghetti-like strands. Drain briefly if the strands contain excess moisture.
8. Combine the spaghetti squash strands with the vegetable and marinara mixture. Gently stir in the ricotta, 1 cup of mozzarella, and 1/4 cup of Parmesan.
9. Transfer the mixture to a lightly greased casserole dish and spread evenly. Sprinkle the remaining 1 cup mozzarella and 1/4 cup Parmesan over the top.
10. Bake for 20–25 minutes, until the casserole is hot and bubbling and the cheese is melted. For a golden top, broil briefly at the end while watching carefully.
11. Let the casserole rest for 5–10 minutes before serving. Garnish with fresh basil or parsley.

Notes

• Add drained chickpeas, cannellini beans, or cooked lentils for extra plant-based protein.
• Mushrooms, broccoli, kale, eggplant, and artichoke hearts can be added for additional vegetables. Cook moisture-rich vegetables before assembling the casserole.
• Cottage cheese can replace the ricotta, and provolone can be used alongside the mozzarella.
• For a vegan version, use dairy-free ricotta, mozzarella, and Parmesan alternatives and check that the marinara is vegan.
• Avoid overcooking the spaghetti squash and drain particularly moist strands before assembling to prevent a watery casserole.
• The casserole can be assembled ahead, covered, and refrigerated until ready to bake. Add a few extra minutes of baking time when cooking it directly from the refrigerator.
• Store leftovers in an airtight container in the refrigerator for 3–4 days. Reheat individual portions in the microwave or cover and warm larger portions at 350°F (175°C).
• Individual portions can be frozen in airtight freezer-safe containers. Thaw overnight in the refrigerator before reheating.

Nutrition

• Serving Size: 1/6 of casserole
• Calories: 360 kcal
• Sugar: 11 g
• Sodium: 780 mg
• Fat: 21 g
• Saturated Fat: 10 g
• Unsaturated Fat: 10 g
• Trans Fat: 0 g
• Carbohydrates: 27 g
• Fiber: 6 g
• Protein: 19 g
• Cholesterol: 48 mg