1. Preheat the oven to 375°F. Lightly grease a 9×13-inch baking dish.

2. Bring a large pot of salted water to a boil. Cook the pasta until just shy of al dente according to package directions. Drain and drizzle lightly with olive oil to prevent sticking.

3. While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add the onion and cook for about 3 minutes until softened.

4. Add the turkey sausage to the skillet. Break it into small pieces and cook until browned and fully cooked through. Drain any excess liquid if needed.

5. In a large mixing bowl, combine the ricotta cheese, egg, 2 cups mozzarella, Parmesan, basil, Italian seasoning, garlic, salt, black pepper, and red pepper flakes.

6. Stir the cooked pasta and sausage into the cheese mixture until evenly coated.

7. Spread about 2 cups of marinara sauce evenly across the bottom of the prepared baking dish.

8. Spoon the pasta mixture into the dish and spread it out evenly. Pour the remaining marinara sauce over the top.

9. Sprinkle the remaining mozzarella cheese evenly over the pasta. Cover loosely with foil.

10. Bake for 20 minutes. Remove the foil and continue baking for another 10 minutes, or until the cheese is melted and bubbling.

11. For a golden top, broil for 2 to 3 minutes if desired.

12. Let the pasta bake rest for 10 minutes before serving. Garnish with extra fresh basil.