Season chicken with garlic powder, onion powder, 1/8 teaspoon salt, and black pepper. Set aside.
Bring 2 skillets sprayed with nonstick spray to medium heat on the stove. In one skillet, cook bell pepper, mushrooms, and onion until softened, 4 to 6 minutes.
In the other skillet, cook chicken until cooked through, about 3 minutes per side.
Cook's Note: Alternatively, use one skillet and cook the veggies and chicken consecutively.
Place crushed tomatoes in a small bowl and season to taste with garlic powder, onion powder, salt, and pepper. Evenly spoon seasoned tomatoes onto chicken in skillet. Tear string cheese into pieces and place over the tomatoes.
Top chicken with vegetables and turkey pepperoni. Cover with a lid and continue to cook until the cheese softens, 1 to 2 minutes.
Plate your pizza-fied chicken and sprinkle with grated topping. Garnish with crushed red pepper and oregano, if using. Enjoy!