Cook pasta in boiling salted water for 1 minute less than package instructions. (Pasta will continue to cook in the oven, so you can undercook it slightly in this step.)

Using a large, deep skillet, add Italian sausage and place over medium-high heat. Cook for 4-5 minutes, stirring occasionally, or until sausage is browned.

Add onions and continue cooking for 4-5 more minutes, or until onions have softened slightly.

Add garlic, basil, Italian seasonings and red pepper flakes; stir until well combined. Continue cooking for 1-2 more minutes.

Add marinara sauce, stir and bring to a simmer.

Preheat oven to 400°F.

Using a large casserole dish, spread ~½ cup of sauce mixture into bottom. Using half of the container of ricotta cheese, drop spoonfuls evenly on top of sauce. Add several more large spoonfuls of sauce on top of the ricotta.

Pour cooked pasta into the casserole dish; top with all of the remaining sauce and ricotta cheese.

Spread shredded mozzarella and grated Parmesan evenly on top of sauce.

Bake for 20 minutes, or until cheese has completely melted.