Preheat oven to 400º. Cut spaghetti squash in half lengthwise and scoop out seeds and membrane. Season lightly with salt and black pepper; bake for about 1 hour, or longer if needed on a baking sheet, cut side down.
In a small bowl combine the ricotta cheese, parmesan cheese, and parsley.
In a large deep sauté pan, heat oil and add onions and garlic; sauté on medium-low for about 3 to 4 minutes, until soft.
Add the sausage and cook, breaking it up into smaller pieces until browned and cooked through.
When cooked, add the crushed tomatoes and add salt and pepper to taste. Reduce heat to low and simmer for 20 to 30 minutes, adding fresh basil at the very end.
When spaghetti squash is cooked, let it cool for about 10 minutes keeping the oven on.
When the spaghetti squash is cool enough to handle, use a fork to remove the flesh, which will come out in spaghetti-looking strands reserving the shells.
Drain the squash on a paper towel to soak up any excess liquid, then toss it with half of the sauce. Place the spaghetti squash back into the 4 shells and place them on a baking sheet.
Top each with the remaining sauce, 1 scoop of ricotta cheese mixture, and a little mozzarella cheese.
Bake in the oven for 20 - 30 minutes or until everything is hot and the cheese is melted.