Heat olive oil in large skillet over medium heat. Add zucchini slices, pepperoni, mushrooms, onions, green peppers. Sprinkle with salt, pepper, and Italian seasoning. Cook 10 minutes or until pepperoni and vegetables start to soften, stirring frequently (so nothing sticks or scorches).
Turn heat to low and cover ingredients with mozzarella then pour tomato sauce over cheese until all is covered. Let simmer 20 minutes or until cheese is completely melted.