Gather and prep all ingredients. Preheat the oven to 375°F.
Heat a large oven safe skillet over medium heat and add sausage and cook for about 5-7 minutes until browned, breaking it up with a spoon.
Stir in onion, bell pepper, mushrooms, and garlic. Cook for 4-5 minutes until softened and fragrant.
Stir in the uncooked rotini pasta, pasta sauce, water, Italian seasoning, red pepper flakes, salt, and pepper. Make sure pasta is mostly submerged in sauce.
Reduce heat to low, cover****, and simmer for 10-12 minutes until pasta is al dente and sauce is mostly absorbed. Make sure to stir occasionally to keep pasta from sticking.
Sprinkle mozzarella evenly over the top. Arrange pepperoni slices and sprinkle with black olives. Bake in the oven for 10-15 minutes uncovered, until the top is browned and the cheese is melted and bubbly.
Let sit for 5 minutes before serving for the casserole to set. Garnish with fresh basil and Parmesan (optional, if you've got them) and enjoy!
*I use sweet Italian sausage because it adds plenty of savory seasoning without making the casserole spicy. Mild or hot Italian sausage will also work. If you use plain ground beef, chicken, or turkey, you may want to add extra seasoning and adjust the salt to taste to make up for the seasonings you’ll lose from swapping out the Italian sausage.**I always use rotini whenever I make this pizza casserole because I love how the sauce clings to every twist! Penne or another similarly sized pasta should also work, but keep in mind that different pasta shapes can cook at different rates. If the pasta isn’t fully cooked, add a splash of water (if needed), cover the skillet, and let it simmer until al dente. If there’s excess liquid and the pasta is already done, simmer uncovered for a few minutes to let it reduce.***You can use a homemade sauce or any preferred brand of pasta sauce here. Even plain tomato sauce or passata with additional seasonings (about 1 teaspoon of Italian seasoning and salt to taste) will work if that’s what you have on hand. ****If your skillet doesn’t have a lid, covering with aluminum foil will do the trick!No large oven-safe skillet? You can cook the sausage, vegetables, pasta, and sauce in a regular skillet, then transfer everything to a baking dish before adding the cheese and toppings. The baking time may be slightly longer because cast iron holds onto heat better than most glass or ceramic baking dishes.