1. Preheat the oven to 400°F and line a baking sheet with parchment paper.

2. Cut each spaghetti squash in half lengthwise, scoop out the seeds, brush the cut sides with olive oil, and season with salt and black pepper.

3. Place the squash halves cut-side down on the baking sheet and roast for 35 to 40 minutes, until tender.

4. While the squash roasts, cook the Italian sausage in a large skillet over medium heat until browned, breaking it into crumbles.

5. Add the diced onion and cook for 3 to 4 minutes, until softened.

6. Stir in the garlic, Italian seasoning, and red pepper flakes, then cook for 30 seconds.

7. Add the cherry tomatoes and spinach, then cook until the tomatoes soften slightly and the spinach wilts.

8. Stir in the marinara sauce and simmer for 2 minutes.

9. Scrape most of the roasted spaghetti squash strands with a fork and mix them into the sausage filling, keeping the shells intact.

10. Spoon the filling back into the squash shells and top with mozzarella and Parmesan.

11. Bake for 10 to 15 minutes, until the cheese is melted and the filling is hot.

12. Rest for 5 minutes, garnish with fresh basil, and serve warm.