Preheat oven to 350°F. Spray a 3-quart baking dish with cooking spray and set aside.
Boil mostaccioli noodles to al dente according to package directions, then drain and rinse under cool water. Return the pasta to the empty pot.
1 pound mostaccioli pasta
In a large skillet set over medium-high heat, brown the Italian sausage, onion, and garlic until cooked through.
1 pound Italian sausage, ½ cup diced onion, 3 cloves garlic
Add the marinara sauce and stir, simmering for 3-4 minutes.
24 ounces marinara sauce
Pour the meat sauce into the pot of pasta and mix well.
In a large bowl, combine the egg, ricotta cheese, oregano, 1 cup mozzarella cheese, and ¼ cup Parmesan cheese.
15 ounces ricotta cheese, 1 large egg, ½ teaspoon oregano, 2 cups shredded mozzarella cheese, ½ cup shredded Parmesan cheese
Place half of the pasta mixture into the prepared baking dish.
Spread the ricotta mixture evenly across the pasta, then top with the remaining pasta mixture. Sprinkle the remaining cheeses on top.
Cover with aluminum foil and bake for 30 minutes.
Remove the aluminum foil and bake an additional 10 minutes, until the cheese is melted. Garnish with grated Parmesan cheese, chopped parsley, and serve.
Freshly chopped parsley