1. Preheat oven to 375° F.
2. Line large baking sheet with parchment paper. Set aside.
3. Cut spaghetti squash in half lengthwise. Remove seeds and membrane.
4. Place spaghetti squash, cut side down, on baking sheet. Bake for 40 to 45 minutes, or until tender.
5. Reduce oven temperature to 350º F.
6. Scrape spaghetti squash flesh into stringy noodles.
7. Lightly coat 4-quart baking dish with spray.
8. Evenly layer half spaghetti squash, half marinara sauce, and half ricotta cheese in baking dish. Repeat with second layer.
9. Evenly top with Parmesan cheese.
10. Bake for 30 to 32 minutes, or until golden brown and bubbly.
11. Garnish with basil (if desired).