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

2. Carefully cut each spaghetti squash in half lengthwise and scoop out the seeds and stringy center.

3. Drizzle the cut sides of the squash with 1 tablespoon of olive oil and season with 0.5 teaspoon kosher salt and 0.25 teaspoon black pepper.

4. Place the squash halves cut side down on the prepared baking sheet and roast for 35 to 45 minutes, or until the flesh is tender and easily shreds into strands with a fork.

5. While the squash is roasting, heat the remaining 1 tablespoon olive oil in a large skillet over medium heat.

6. Add the chopped onion and cook for 3 to 4 minutes, stirring often, until softened and translucent.

7. Stir in the sliced cremini mushrooms and cook for 6 to 8 minutes, until they release their moisture and turn golden brown around the edges.

8. Add the minced garlic to the skillet and cook for 30 seconds, just until fragrant.

9. Stir in the fresh baby spinach, adding it in batches and letting it wilt before adding more.

10. Season the vegetables with the remaining 0.5 teaspoon kosher salt, 0.25 teaspoon black pepper, Italian seasoning, and crushed red pepper flakes if using.

11. Pour the heavy cream or half and half into the skillet, scraping up any browned bits from the bottom of the pan.

12. Bring the mixture to a gentle simmer and cook for 2 to 3 minutes to slightly thicken the sauce.

13. Stir in the grated Parmesan cheese until it melts and the sauce becomes smooth and creamy. Remove the skillet from the heat.

14. When the roasted spaghetti squash is cool enough to handle, flip the halves over and use a fork to scrape the flesh into long strands, leaving the shells intact to form “boats.”

15. Add the squash strands to the skillet with the creamy spinach and mushroom mixture and toss gently until everything is evenly coated. Taste and adjust seasoning with additional salt or pepper if needed.

16. Arrange the empty squash shells on the baking sheet and divide the creamy squash mixture evenly among the four halves, mounding it slightly.

17. Sprinkle the tops of the stuffed squash with shredded mozzarella and a little extra grated Parmesan.

18. Return the baking sheet to the oven and bake for 10 to 12 minutes, or until the cheese is melted and bubbling. For extra browning, broil for 1 to 2 minutes, watching closely.

19. Remove from the oven and let the squash rest for 5 minutes to set slightly.

20. Garnish with chopped fresh parsley and an extra pinch of black pepper before serving hot right from the squash shells.