Spray 10-inch (25 cm) ovenproof non-stick skillet with cooking spray. Heat skillet over medium-high heat. Add potatoes, onion and green pepper; cook, stirring frequently, until tender.
Pour egg mixture over potato mixture. As eggs set around edge of skillet, with spatula, gently lift cooked portions to allow uncooked egg to flow underneath. Cook until bottom is set and top is almost set. Arrange pepperoni slices over frittata.
Place skillet under preheated broiler for 2 minutes. Sprinkle tomato and cheese over frittata. Broil for 1 or 2 minutes to melt cheese. Loosen frittata around edge of skillet, then slide onto plate. Cut into wedges.
Instead of broiling, cover skillet with a lid and cook for a few minutes longer on the stovetop.