Combine the first 7 ingredients in a medium bowl; set aside.
Melt butter in a 10-inch cast-iron or nonstick skillet over medium heat. Add onion; sauté 5 minutes. Add potatoes; cook 9 minutes or until lightly browned, stirring occasionally. Pour egg mixture over onion mixture. Arrange bell pepper slices on top of frittata. Cook 7 minutes or until set. Sprinkle with cheese. Wrap handle of pan with foil.
Broil 5 minutes or until cheese is lightly browned. Cut into 4 wedges.