Preheat and prep: Set your oven to 375°F (190°C). Cook 1 lb of pasta according to package directions until al dente.
Cook the sausage: In a large skillet, cook 1 lb of Italian sausage and 1 diced onion over medium-high heat until browned.
Make the sauce: Stir in 3 cloves of minced garlic, 1 tsp of Italian seasoning, and ¼ tsp of red pepper flakes. Cook for one minute.
Simmer the sauce: Pour marinara sauce (24 oz) into the skillet. Bring it to a simmer and then reduce the heat to low.
Mix the cheese: In a bowl, combine ricotta(15 oz), ½ cup of Parmesan, 1 large egg, and 1 ½ cups of mozzarella.
Combine everything: In a large bowl, mix the cooked pasta, the meat sauce, the cheese mixture, and ¼ cup of fresh basil.
Assemble the bake: Pour the mixture into your 9x13-inch baking dish. Top with the remaining 1 ½ cups of mozzarella cheese.
Bake the casserole: Bake for 25-30 minutes, or until the cheese is melted and bubbly. Let it rest for 10 minutes and add more additiona chopped basil before serving.