Boil pasta according to package directions, but undercook the pasta for about 2 minutes. This will be just slightly under al dente. The rigatoni will continue to cook while in the oven and this will prevent it from becoming overcooked and mushy.
1 pound rigatoni pasta
While the pasta is cooking, add 1-2 tablespoons of olive oil to a saute pan. Add onions and cook over medium heat.
1-2 tablespoons olive oil, ½ cup diced onion
When the onions have just begun to soften, add the Italian sausage. Use a spoon to break the sausage into small bits. Cook until the sausage is fully cooked.
1 pound Italian sausage
Move sausage and onions to one side and add the garlic. Cook for about 30 seconds, just until fragrant.
1 teaspoon minced garlic
Remove from heat and drain off any excess grease.
Add sausage to the marinara sauce.
1 jar marinara sauce
When pasta is done, drain (but do not rinse). Add pasta to the marinara sauce.
In a separate bowl, combine ricotta with Italian herbs, salt and pepper.
Layer half the pasta mixture into a greased deep-dish casserole.
Add half of the ricotta mixture and sprinkle with half of the shredded mozzarella.
2 ½ cups grated mozzarella
Repeat with the remaining pasta mixture, then ricotta and mozzarella. Finish by topping with the grated parmesan.
¼ cup grated parmesan
Bake covered in a preheated 375°F oven for 20 minutes. Then uncover dish and continue baking for 15-20 minutes until the casserole is bubbly and the top is browned.
If you want an even browner cheesy topping, pop under the broiler for 2-3 minutes. Keep a close eye on it so it doesn't burn.
Allow casserole to rest for 10 minutes before serving.
Top with fresh parsley or basil for a garnish, if desired.
Notes
To make casserole ahead of time
Make as directed but instead of baking, cover and refrigerate for up to 24 hours.
When ready, remove from refrigerator for at least 30 minutes.
Bake in preheated 375°F oven for 40 minutes with cover and then for 25-35 minutes without cover until fully heated through.
Cooking Tips
Use a large deep-dish casserole dish that will hold 3-4 quarts.
Bake on a cookie sheet to help catch any drips or overflow.
Allow baked pasta to rest for 10 minutes before serving.