For the pasta and sauce:
1 tablespoon olive oil
1 pound uncooked Italian sausage, casings removed
2 medium bell peppers, diced
1 medium yellow onion, diced
2 cloves garlic, finely chopped
1/2 teaspoon red pepper flakes (optional)
4 cups marinara sauce (about 36 ounces)
1/2 teaspoon kosher salt, plus more for seasoning
Freshly ground black pepper
1 pound dried ziti or penne pasta
1 cup whole-milk ricotta cheese
1/2 cup finely grated Parmesan cheese, divided
1/4 teaspoon kosher salt
8 ounces shredded mozzarella and/or provolone cheese (about 2 cups)
Arrange a rack in the middle of the oven and heat to 375°F. Meanwhile, bring a large pot of heavily salted water to a boil over medium-high heat and make the sauce.
Heat the oil in a large pot or Dutch oven over medium heat until shimmering. Add the sausage and cook, breaking up the meat into smaller pieces with a wooden spoon, until cooked through, 8 to 10 minutes.
Add the bell peppers, onion, garlic, and pepper flakes; season with salt and pepper; and cook, stirring occasionally, until softened, about 10 minutes. Add the marinara and 1/2 teaspoon salt, stir to combine, and bring to a simmer. Remove from the heat.
When the water is boiling, add the pasta and cook until just tender. While the pasta is cooking, stir the ricotta, 1/4 cup of the Parmesan, and salt together in a small bowl.
When the pasta is ready, drain it in a colander or strainer. Return the pasta to the now-empty pot. Add half of the sauce (about 3 generous cups) and stir until the pasta is well-coated. Taste and season with salt and pepper as needed. Add the ricotta mixture and fold it in, leaving big pockets of ricotta here and there.
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 or provolone. Top with the remaining pasta in an even layer.
Pour the remaining sauce over the pasta and spread into an even layer. Sprinkle the surface evenly with the remaining mozzarella or provolone and remaining 1/4 cup Parmesan.
Bake uncovered until the edges are bubbling and the cheese is completely melted and browned in spots, about 30 minutes. Remove the baking dish to a wire rack and let cool 15 minutes before serving.