For the Meatballs:

1. Preheat Oven: Preheat your oven to 375°F (190°C) for baking the meatballs.
2. Make the Meatball Mixture: In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, parsley, garlic, oregano, salt, and pepper. Add the milk if you want a softer meatball. Mix gently until everything is well combined (don’t overmix).
3. Shape the Meatballs: Roll the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them on a baking sheet lined with parchment paper.
4. Bake the Meatballs: Bake the meatballs in the preheated oven for 20-25 minutes, or until golden brown and cooked through. Once done, remove from the oven and set aside.
5. For the Sauce:
6. Make the Marinara Sauce: While the meatballs are baking, heat olive oil in a large saucepan over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute until fragrant.
7. Simmer the Sauce: Stir in the crushed tomatoes, oregano, basil, sugar (if using), salt, and pepper. Bring the sauce to a simmer, then reduce the heat and let it simmer for 15-20 minutes to let the flavors meld together.
8. Add the Meatballs to the Sauce: Once the sauce is ready, add the baked meatballs to the saucepan. Stir to coat the meatballs in the sauce and let them simmer for another 5-10 minutes.
9. For the Sub:
10. Prepare the Sub Rolls: While the meatballs are simmering, split the sub rolls and place them on a baking sheet. Lightly toast them in the oven for 5 minutes for a crispy texture if desired.
11. Assemble the Meatball Parm Subs: Spoon a few meatballs and plenty of marinara sauce into each sub roll. Top with shredded mozzarella cheese and a sprinkle of grated Parmesan.
12. Melt the Cheese: Place the assembled subs under the broiler for 2-3 minutes, or until the cheese is melted, bubbly, and slightly golden.
13. Serve: Remove the subs from the oven, garnish with fresh basil or parsley (if desired), and serve immediately with a side of chips or a salad.