Roast squash halves cut-side down at 400°F for 35–40 minutes until tender; scrape strands with a fork and set aside.
While squash roasts, sauté sausage in olive oil until browned; remove and set aside.
In same pan, cook onion and pepper until soft, add garlic for 30 seconds.
Return sausage to pan, stir in marinara and Italian seasoning; simmer 5 minutes.
Mix sauce with squash strands and ricotta in a large bowl; season to taste.
Transfer to a baking dish, top with mozzarella and Parmesan, then bake at 375°F for 15–20 minutes until bubbly.
Garnish with parsley and let rest 5 minutes before serving.