Preheat your oven to 400°F (200°C).

Cut the spaghetti squash in half lengthwise, scoop out the seeds, drizzle with olive oil, and sprinkle with salt and pepper.

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

In a skillet, heat olive oil and sauté the diced onion until translucent, then add garlic and sliced mushrooms until browned.

Stir in fresh spinach and cook until wilted, then remove from heat.

In a mixing bowl, combine cream cheese, Parmesan cheese, nutmeg, and the sautéed mixture. Season with salt and pepper.

Scrape the flesh of the roasted spaghetti squash into strands, mix with the creamy filling, and spoon back into each squash half. Top with mozzarella cheese and return to the oven.