Step 1

Heat the oven to 375°F. Arrange a rack in the middle of the oven and heat to 375°F. Cook the pasta and sauce while the oven is heating.

Step 2

Boil the water. Bring a large pot of heavily salted water to a boil over medium-high heat. Meanwhile, make the sauce.

Step 3

Cook the onions and garlic. Heat the oil in a large pot or Dutch oven over medium heat until shimmering. Add the onion, garlic, and pepper flakes, season with salt and pepper, and cook, stirring occasionally, until softened, about 12 minutes.

Step 4

Simmer the sauce. Add the marinara and 1 teaspoon salt, stir to combine, and bring to a simmer. Remove from the heat.

Step 5

Cook the pasta. When the water is boiling, add the pasta and cook until just tender. While the pasta is cooking, make the ricotta filling.

Step 6

Make the ricotta filling. Stir the ricotta, 1/4 cup of the Parmesan, and salt together in a small bowl; set aside.

Step 7

Drain the pasta. When the pasta is ready, drain it in a colander or strainer.

Step 8

Sauce the pasta. Return the pasta to the now-empty pot. Add half of the sauce (about 3 cups) and stir until the pasta is well-coated. Taste and season with salt and pepper as needed.

Step 9

Fold in the ricotta. Add the ricotta mixture to the pasta and fold it in, leaving big pockets of ricotta here and there.

Step 10

Layer the pasta. Transfer half of the pasta mixture into a 9x13-inch baking dish and spread into an even layer. Evenly sprinkle with half of the mozzarella. Top with the remaining pasta in an even layer.

Step 11

Layer the sauce and cheese. Pour the remaining sauce over the pasta and spread into an even layer. Sprinkle the surface evenly with the remaining mozzarella and remaining 1/4 cup Parmesan.

Step 12

Bake the casserole. Bake uncovered until the edges are bubbling and the cheese is completely melted and browned in spots, about 30 minutes.

Step 13

Cool the casserole. Remove the baking dish to a wire rack and let cool 15 minutes before serving.