Prepare the spaghetti squash: Preheat the oven to 400°F (200°C). Cut the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the inside of each half with olive oil, then sprinkle with salt and pepper.

Place the squash halves cut side down on a baking sheet lined with parchment paper. Roast for 40-45 minutes, or until the flesh is tender and easily shredded with a fork.

Cook the sausage filling: While the squash is roasting, heat a large skillet over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned and fully cooked (about 5-7 minutes).

Add the chopped onion and garlic to the sausage and sauté for 2-3 minutes, or until softened. Stir in the marinara sauce and red pepper flakes (if using). Let the mixture simmer for 5 minutes. Taste and adjust seasoning with salt and pepper.

Stuff the squash: Once the squash is done roasting, use a fork to gently scrape the flesh into strands, leaving a border around the edges.

Spoon the sausage mixture into the spaghetti squash halves, pressing it down slightly.

Sprinkle the shredded mozzarella and grated Parmesan cheese evenly over the top.

Return the stuffed squash to the oven and bake for another 10 minutes, or until the cheese is melted and bubbly.

Serve: Remove from the oven and garnish with fresh basil or parsley. Serve immediately and enjoy!