Brown the ground beef or mild Italian sausage with the diced onion in a skillet over medium heat for 8 to 10 minutes, breaking the meat into small pieces. Drain any excess fat, then stir in the minced garlic, Italian seasoning, salt, and black pepper.

Cook the rotini separately according to the package directions, stopping when it is just al dente. Drain it well so the casserole does not become watery or overly soft during slow cooking.

Lightly grease a 5- to 6-quart slow cooker. Add the cooked pasta, meat mixture, pizza sauce, tomato sauce, optional drained diced tomatoes, and about half of the sliced pepperoni.

Stir the ingredients gently until the pasta is coated and the meat and pepperoni are distributed throughout. Cover and cook on LOW for 2 to 3 hours or on HIGH for 1 to 1 1/2 hours, until hot throughout.

Sprinkle the mozzarella and optional cheddar or Italian-blend cheese over the casserole. Arrange the remaining pepperoni over the cheese, cover again, and cook for another 10 to 15 minutes, until the cheese is fully melted.

Spoon the casserole into bowls and serve hot. Let it stand for a few minutes before serving so the sauce settles slightly and the cheese remains pleasantly stretchy.