Break string cheese into thirds and place in a blender or food processor. Blend at high speed until cheese takes on a shredded or grated consistency. (If you do not have a food processor, tear string cheese into pieces and roughly chop.) Set aside.
Place crushed tomatoes in a small bowl and season with garlic powder, onion powder, salt and black pepper.
Bring a skillet sprayed with nonstick spray to medium heat. Cook bell pepper, mushrooms and onion until softened, 4 to 6 minutes.
Remove veggies. Remove skillet from heat, clean and re-spray.
Prepare chicken breast fillet according to microwave instructions.
Place warmed chicken in prepared skillet and return to medium heat. Evenly top with seasoned tomatoes, prepared string cheese, veggies and turkey pepperoni.
Cover and continue to cook until cheese softens, 1 to 2 minutes. Sprinkle with Parmesan cheese and serve.