1. Start by preheating your oven and cutting the spaghetti squash in half lengthwise. Scoop out the seeds and place the halves cut-side down on a baking sheet. Roast the squash until tender, then use a fork to gently shred the flesh into strands.

2. While the squash is baking, brown the Italian sausage in a skillet over medium heat, breaking it up as it cooks. Add the diced onion and minced garlic, sautéing until fragrant. Then, stir in the chopped spinach and cook until wilted. Finally, mix in the ricotta, mozzarella, and Parmesan cheeses, seasoning with salt and pepper to taste.

3. Once the spaghetti squash is ready, gently combine the sausage-spinach filling with the squash strands, then spoon the mixture back into the squash halves. Top with a sprinkle of additional mozzarella cheese and bake until the cheese is melted and bubbly. Serve hot and enjoy the perfect balance of flavors and textures.