1. Preheat the oven to 400°F (200°C).

2. Cut the spaghetti squash in half lengthwise and scoop out the seeds.

3. Drizzle the inside of each half with olive oil, and season with salt and pepper.

4. Place the squash halves cut side down on a baking sheet and roast for 30-35 minutes, until tender.

5. In a skillet over medium heat, add olive oil and sauté the onion until translucent.

6. Add the garlic and mushrooms, cooking until the mushrooms are browned and released their moisture.

7. Stir in the spinach and cook until wilted. Remove from heat.

8. In a bowl, combine the cream cheese, Parmesan cheese, nutmeg, and the cooked mushroom-spinach mixture. Mix well and season with salt and pepper.

9. Once the squash is done, use a fork to scrape the flesh into spaghetti-like strands.

10. Mix the squash strands with the creamy filling until well combined.

11. Spoon the mixture back into the squash halves and top with shredded mozzarella cheese.

12. Return to the oven and bake for an additional 10-15 minutes, until the cheese is melted and bubbly.

13. Serve warm.