Reserve 10-12 slices of pepperoni. Coarsely chop the remaining pepperoni.
Cook rotini pasta according to package directions. Drain; set aside.
Heat the oil in a large skillet over medium-high heat. Saute onions for several minutes until tender. Add the garlic; saute one minute.
Add the soup, milk, Parmesan cheese, 1 cup of the mozzarella cheese, oregano and black pepper and bring to a boil.
Stir in the chicken, chopped pepperoni, rotini and tomatoes. Sprinkle with remaining mozzarella cheese. Top with the reserved pepperoni slices. Reduce heat to low; cover and cook until mixture is hot.
Notes
1 can (10 oz) hot diced tomatoes with habaneros or chopped green chiles can be substituted for the fresh tomatoes. I drained off the extra liquid before adding into recipe.