Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

Prepare the squash: Cut spaghetti squash in half lengthwise and scoop out seeds. Drizzle with olive oil, sprinkle with salt and pepper, and place cut-side down on the baking sheet.

Roast the squash for 35–40 minutes, or until the flesh is tender and can be shredded with a fork.

Cook the sausage: While the squash is roasting, heat a skillet over medium heat. Add sausage and cook until browned, breaking it apart with a spoon. Add diced onion and cook until softened, about 5 minutes. Stir in garlic and cook for another 1–2 minutes.

Mix in sauce: Add marinara sauce and Italian seasoning. Stir to combine, then remove from heat.

Assemble: Once the squash is cooked, flip them over and use a fork to gently shred the flesh into spaghetti-like strands, keeping them inside the shell. Spoon the sausage mixture evenly into each squash half. Top with mozzarella and Parmesan.

Bake again: Return to oven and bake for 10–15 minutes, or until cheese is melted and bubbly. Broil for 2–3 minutes if you want a golden top.

Serve: Sprinkle with fresh parsley if desired and serve warm.