Pound the chicken as thin as possible without tearing it. Season both sides of the chicken with the seasonings and place it on a foil-lined baking sheet. Spread the pizza sauce on the top of the chicken. Arrange the remaining topping ingredients in this order: pepperoni, mushrooms, sausage, cheese, peppers and then the onions. Lightly sprinkle with a little more Italian seasoning and garlic powder. Bake at 400º 10 minutes. Do not overcook.