1. Preheat the Oven: Set your oven to 400°F (200°C).
2. Cook the Pasta: Boil the spaghetti, cooking it 1-2 minutes less than recommended. Drain and set aside.
3. Prepare the Meat Sauce: In a Dutch oven, brown the Italian sausage over medium heat, breaking it up as it cooks. Drain any excess grease. Add the diced onion and cook until translucent, then stir in the minced garlic and red pepper flakes, cooking for another 30 seconds. Pour in the 4 cups of marinara sauce, season with salt and black pepper, and let it simmer for 5 minutes.
4. Prepare the Cheese Mixture: In a large bowl, mix together the ricotta cheese, Boursin cheese and basil leaves.
5. Combine Pasta and Sauce: In a large bowl, mix the cooked spaghetti with the meat sauce.
6. Build the Layers: Spread 1 cup of the marinara sauce evenly across the bottom of a 9x13 inch baking dish. Pour half of the pasta mixture into the baking dish. Dollops the cheese mixture over the noodles and spread evenly. Sprinkle with half of the shredded mozzarella and Parmesans cheese. Add the remaining pasta on top, followed by the rest of the marinara sauce, mozzarella and Parmesan cheese.
7. Bake: Cover with foil (if desired) and bake for 30-35 minutes, or until golden brown and bubbly.
8. Serve: Let it cool for a few minutes before serving. Enjoy your delicious baked rigatoni!

Feel free to customize with your favorite herbs or additional toppings!