Preheat oven to 375˚F.

Prick the squash all over with a sharp paring knife. Place on a rimmed baking sheet and bake until tender when pierced with a knife, about 1 hour 20 minutes, flipping halfway through the cooking time. Remove from oven and set aside to cool. When cool enough to handle, slice in half and scoop out the seeds. Use a fork to scrape out the flesh in long strands. Place in a large bowl and set aside.

Increase oven heat to 400˚F.

In a large sauce pan over medium heat, sauté the onion in olive oil for 3-4 minutes, until it starts to soften and becomes translucent. Season with salt and pepper and add the garlic. Continue to cook for 1-2 minutes. Add the sausage and cook until mostly cooked through, crumbling it with the back of a wooded spoon. Stir in the mushrooms and cook for 5 minutes, then pour in the crushed tomatoes and cook until heated through.

Pour the sauce over the spaghetti squash and toss to combine. Transfer to a 9x13-inch baking dish. Top with dollops of ricotta, then the shredded mozzarella and grated Parmesan.

Bake for 20-25 minutes, until the cheese is melted and lightly browned, and the casserole is heated through. Allow to rest for 5 minutes before serving.