Beat eggs with water; season with salt and pepper. Set aside. Heat an 8 inch ( 20 cm) non stick skillet over medium high heat. Saute potatoes, onion and green pepper until tender. Pour egg mixture over potato mixture. As mixture sets at edges, with spatula, gently lift cooked portion to allow uncooked egg to flow underneath. Cook until the bottom is set and the top is almost set. Arrange pepperoni slices over frittata. Broil for 2 minutes; sprinkle tomato and cheese over frittata. Broil for additional 1 to 2 minutes or until eggs are completely set and cheese is melted.