Preheat oven to 400ºF.
Cut spaghetti squash in half lengthwise and scoop out seeds and membrane. Season lightly with salt and black pepper, place cut side down on a baking sheet, and bake about 1 hour or until tender.
If using the microwave instead, cut squash in half lengthwise, scoop out seeds and fibers, place cut side down on a microwave-safe dish, cover, and microwave 8–9 minutes or until soft; this method is faster but will produce more watery squash.
While the squash cooks, combine the ricotta cheese, 2 tbsp grated parmesan, and 1 tbsp chopped parsley (or basil) in a small bowl.
In a large deep sauté pan, heat 1 tsp olive oil over medium-low, add the 1/2 onion and 3 cloves garlic, and sauté about 3 to 4 minutes until soft.
Add the 14 oz Italian chicken sausage, breaking it up into smaller pieces, and cook until browned and cooked through.
When the sausage is cooked, add the 14 oz crushed tomatoes, season with salt and fresh pepper to taste, add the 1 bay leaf, cover, reduce heat to low, and simmer 20 to 30 minutes; stir in the 2 tbsp chopped basil at the very end.
When the spaghetti squash is cooked, let it cool for about 10 minutes while keeping the oven on. If you microwaved the squash, preheat the oven to 400ºF now.
When cool enough to handle, use a fork to scrape the flesh into spaghetti-like strands, reserving the shells.
Drain the scraped squash on paper towels to soak up excess liquid, then toss the strands with half of the sauce. Return the strands to the 6 shells and arrange them on a baking sheet.
Top each shell with the remaining sauce, 1 tbsp of the ricotta-parmesan mixture, and 2 tbsp shredded mozzarella. Bake in the oven for 20 to 30 minutes or until everything is hot and the cheese is melted.