Preheat your oven to 400°F (200°C). Slice the spaghetti squash in half lengthwise and scoop out seeds. Drizzle olive oil, season with salt and black pepper, and roast for 35–40 minutes until tender.

In a skillet, heat olive oil over medium heat. Cook the Italian sausage for 6–8 minutes until browned. Add onion, bell peppers, and garlic, sauté for 5-7 minutes until tender.

Stir in marinara sauce, Italian seasoning, and red pepper flakes. Let simmer for 5 minutes.

Once the spaghetti squash is roasted, scrape strands into a bowl, drain excess moisture. Mix with sausage mixture and fold in mozzarella and Parmesan.

Spread the mixture in a greased casserole dish. Top with remaining cheeses.

Bake at 375°F (190°C) for 20 minutes covered. Remove foil and bake for an additional 10 minutes until golden and bubbly.

Let cool for 10 minutes, garnish with fresh herbs, and serve.