Preheat oven to 400°F (200°C).
Brush acorn squash halves with olive oil and season with salt and pepper.
Place squash cut-side down on a baking sheet and roast for 25 to 30 minutes until tender.
Heat a skillet over medium heat and cook Italian sausage until browned.
Add diced onion, celery, and garlic to the skillet and sauté for 3 to 4 minutes until softened.
Stir in thyme, sage, paprika, cooked rice or quinoa, Parmesan, salt, and pepper and mix well.
Flip the roasted squash halves over and fill each with the sausage mixture.
Top each filled squash with shredded mozzarella cheese.
Bake in the oven for 10 to 12 minutes until the cheese melts.
Garnish with parsley or chili flakes and serve warm.