Preheat your oven to 350°F (175°C). This ensures that your casserole bakes evenly.
In a large pot, bring salted water to a boil. Add the rotini pasta and cook according to package instructions until al dente. Drain the pasta in a colander and set aside.
In the same pot, add the bulk Italian sausage over medium heat. Cook until browned and fully cooked through, breaking it up with a spatula as it cooks. If you’re opting for a vegetarian version, feel free to skip this step.
In a large mixing bowl, combine the cooked pasta, marinara sauce, cooked sausage (if using), diced green bell pepper, pepperoni, and sliced black olives. Stir in two cups of mozzarella cheese until everything is well combined.
Grease your 9x13-inch baking dish with cooking spray or olive oil. Pour the pasta mixture into the dish, spreading it out evenly. Top with the remaining one cup of mozzarella cheese.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 25 minutes, then remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Once baked, allow the casserole to cool for a few minutes before slicing. Serve warm and enjoy every cheesy, savory bite!
