Preheat oven to 350 degrees. Spray an 8" X 8" baking pan with nonstick spray.
Pound chicken to 1/2-inch thickness. Season with salt and black pepper. Bring an extra-large skillet sprayed with nonstick spray to medium heat. Cook for about 4 minutes per side, until cooked through. Transfer to a cutting board.
Meanwhile, in a large bowl, combine cream cheese with 1/4 teaspoons garlic powder and 1/4 teaspoons onion powder. Stir until smooth and uniform.
Remove skillet from heat; clean, if needed. Re-spray, and bring to medium-high heat. Add mushrooms and bell pepper, and sprinkle with remaining 1/4 teaspoons garlic powder and 1/4 teaspoons onion powder. Cook and stir until slightly softened and lightly browned, about 6 minutes.
Transfer skillet contents to the bowl with the cream cheese mixture. Mix thoroughly.
Chop chicken, and evenly place in the baking pan.
Evenly top chicken with creamy veggies, marinara sauce, mozzarella, and chopped pepperoni.
Bake until entire dish is hot and bubbly and mozzarella has melted, 20 - 25 minutes.