Preheat the oven to 375 degrees and bring a large pot of salted water to a boil. Cook the ziti until al dente, then drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add the sausage and cook until browned, breaking it up with a spoon as it cooks.
Add the onion and bell peppers to the skillet. Cook for 5 minutes until the vegetables soften. Stir in the garlic, Italian seasoning, and salt, then cook for 1 minute more.
Pour the marinara sauce into the skillet and simmer for 3 minutes. Remove from heat and stir in the cooked ziti until evenly coated.
Spread half the pasta mixture in a 9x13 baking dish. Dollop the ricotta over the top and sprinkle with half the mozzarella and Parmesan. Add the remaining pasta mixture and finish with the rest of the mozzarella and Parmesan.
Bake for 20 minutes until the cheese is melted and the edges are bubbly. Let the dish rest for 5 minutes before serving.