Step 1

In a large skillet, heat oil over medium-high heat. Add the onion. Cook, stirring frequently, until just starting to turn translucent, about 5 minutes.

Step 2

Add the sausage and garlic. Cook, breaking up the sausage with the back of a spoon, until browned and no longer pink.

Step 3

Stir in marinara, tomatoes, Italian seasoning, and red pepper flakes. Bring to a simmer. Reduce heat to medium-low and continue simmer, stirring occasionally for 30 minutes, or until thickened. Add salt and pepper to taste.

Step 4

While sauce simmers, preheat the oven to 350F.

Step 5

Bring a large pot of water to a boil. Add 1 teaspoon salt per quart of water. Add the pasta and cook until almost tender. (You want it al dente as it will continue cooking when baking.) Drain and rinse with cold water. Return to the same pot.

Step 6

To the pasta, add 2 cups tomato sauce. Stir until well coated.

Step 7

Spoon some of the tomato sauce into a 13x9-inch or 2 1/2-quart casserole dish, just enough to lightly cover the bottom of the dish. Pour half of the pasta on top.

Step 8

Spoon half of the remaining sauce over the pasta. Sprinkle half of the parmesan, dot half of the ricotta, and sprinkle half of the mozzarella over the pasta. Repeat with remaining pasta, sauce, and cheeses. Place on a rimmed baking sheet.

Step 9

Bake for 40 minutes, or until bubbling around the edges and the top is browned. Let cool for 10 minutes before serving.