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

Prepare the squash: Cut the spaghetti squash in half lengthwise and scoop out the seeds. Brush the insides with olive oil and season with salt and pepper. Place cut side down on the baking sheet.

Roast for 35–40 minutes, or until the squash is tender and the strands pull away easily with a fork.

Cook the sausage filling: While the squash is baking, heat a skillet over medium heat. Add the sausage and cook until browned, breaking it apart with a spoon. Add the diced onion and garlic, cooking until softened.

Add marinara and seasoning: Stir in the marinara sauce and Italian seasoning. Simmer for 5–7 minutes to blend flavors. Remove from heat.

Assemble: Flip the roasted squash halves over. Use a fork to loosen the spaghetti-like strands. Spoon the sausage mixture into each squash half and gently mix with the strands.

Top with cheese: Sprinkle mozzarella and Parmesan cheese over each filled squash half.

Bake again: Return to the oven for 10–12 minutes, or until the cheese is melted and bubbly. Broil for 2 minutes if you want a golden top.

Garnish and serve: Remove from the oven, garnish with chopped parsley if desired, and serve hot.