Preheat oven to 400°F (200°C). Place squash halves cut-side up on a baking sheet. Drizzle with a little olive oil, sprinkle with salt and pepper, and roast for 15 minutes while preparing the filling.

In a large skillet over medium heat, add 1 tablespoon olive oil. Add sausage and cook, breaking it apart with a spoon, until browned and cooked through, about 5–6 minutes.

Add onions and mushrooms to the skillet. Cook for 5–7 minutes until softened and any liquid from the mushrooms has evaporated.

Stir in garlic, thyme, salt, and black pepper. Cook for 1 minute until fragrant. Remove from heat and stir in breadcrumbs and parmesan cheese.

Remove partially roasted squash from the oven. Spoon the sausage and mushroom mixture evenly into each squash half.

Return to oven and bake for an additional 15–20 minutes, or until squash is tender and filling is lightly golden on top.

Let cool for 5 minutes before serving. Enjoy warm as a main dish.