Preheat oven to 400°F (200°C).
Brush acorn squash halves with olive oil and place cut-side down on a baking sheet.
Roast squash for 30–35 minutes until tender.
Heat a large skillet over medium heat.
Add sausage and cook, breaking it up, until browned.
Add onion, garlic, and bell pepper and sauté until softened.
Stir in spinach, salt, black pepper, and dried sage.
Remove skillet from heat and mix in breadcrumbs and Parmesan cheese if using.
Turn roasted squash cut-side up and fill each half with sausage mixture.
Return stuffed squash to oven and bake for 10 minutes.
Remove from oven and serve warm.