Season the duck breasts liberally with salt and pepper. Heat 2 tablespoons of the oil over high heat in a large skillet and sear breasts quickly on both sides, but not past rare. Remove duck and set aside. Add remaining oil and onions. Reduce heat to low and cook onions for 4 to 5 minutes. Add brown sugar and stir to coat. Cook until sugar has melted and has coated the onions, about 3 to 4 minutes more. Add garlic and next 4 ingredients and simmer until liquid is reduced to about 1/4 cup. Return duck breasts to the pan, cover pan and heat duck breasts to medium-rare. Remove duck and slice thinly at a diagonal. Mound with onions and peppers in pan and top with cheese until melted. Spread mayonnaise on rolls and add lettuce and tomato. Mound duck, peppers, onions and cheese onto tomato.