01 - Preheat oven to 375°F and lightly grease a 9x13 inch baking dish.
02 - Bring salted water to a boil and cook ziti until just al dente, about 1-2 minutes less than package instructions. Drain and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until translucent, about 4 minutes. Add minced garlic and diced red bell pepper and sauté for 2 minutes.
04 - Add plant-based sausage crumbles to the skillet and cook until lightly browned, about 4 minutes.
05 - Stir in crushed tomatoes, tomato sauce, dried Italian herbs, basil, red pepper flakes (if using), salt, and black pepper. Simmer for 10 minutes, stirring occasionally.
06 - In a large mixing bowl, combine cooked pasta, half of the mozzarella, half of the Parmesan, all of the ricotta, and half of the prepared sauce. Mix gently to combine.
07 - Spread half of the pasta mixture evenly in the prepared baking dish. Top with the remaining sauce, then sprinkle with the remaining mozzarella and Parmesan cheeses.
08 - Cover the dish with foil and bake for 20 minutes. Remove foil and bake an additional 10 to 15 minutes until cheese is golden and bubbly.
09 - Allow to rest for 10 minutes before serving. Garnish with fresh basil leaves.