Preheat the oven to 375 degrees. Bring a large pot of salted water to a boil and cook the ziti until just al dente. Drain and set aside.

In a large skillet over medium heat, cook the Italian sausage, breaking it up with a spoon, until browned. Add the diced onion and sliced bell peppers. Cook for 5 minutes until the vegetables soften.

Stir the marinara sauce, Italian seasoning, salt, and black pepper into the skillet. Simmer for 3 minutes, then remove from heat.

In a large bowl, combine the cooked ziti with the sausage and pepper mixture. Fold in the ricotta cheese until evenly distributed.

Transfer half the pasta mixture to a greased 9x13 baking dish. Sprinkle with half the mozzarella and Parmesan. Add the remaining pasta mixture and top with the rest of the cheeses.

Cover with foil and bake for 20 minutes. Remove the foil and bake 10 more minutes until the cheese is bubbly and lightly browned.