Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
- Brush the cut sides of the acorn squash with 1 tbsp olive oil, season with salt and pepper, and place cut side down on the baking sheet. Roast for 25–30 minutes until fork-tender.
- Meanwhile, heat 2 tbsp olive oil in a skillet over medium heat. Add sausage and cook until browned, breaking into crumbles.
- Add onion and celery; cook until softened. Stir in garlic and cook for 1 minute.
- Add spinach and cook until wilted. Stir in cooked rice or quinoa, Parmesan cheese, and sage. Season with salt and pepper to taste.
- Remove squash from the oven, turn cut side up, and fill each half generously with the sausage mixture.
- Return to the oven and bake for 10–15 minutes until heated through. Serve hot.