Place the butter in a flameproof nonstick skillet, 5 inches in diameter. Melt butter over moderately high heat, then add the onions. Saute, stirring occasionally, until the onions are golden brown and quite soft, about 10 minutes.
Add beaten eggs to pan. Cook slowly for 10 minutes, or until the base of the frittata is well set. Pass the frittata under a broiler, briefly to set the eggs on top. Slide it out from pan onto plate or countertop, and let cool for 15 minutes.
Cut the frittata into 6 portions that fit snugly on the rolls. Place each portion on half of a roll. Top each with 2 slices of bacon and a third of fresh roasted red pepper. Close sandwiches with top of rolls.