Place duck breasts on a flat surface. Press your hand over each breast and cut into 2 pieces by slicing through the middle of the breast, between your hand and the flat surface. The idea is to have 2 thin pieces of duck breast. If desired, you can lightly pound meat with a mallet so that it is of even thickness. Season sliced duck with salt and pepper. Fry the onion in butter in a large skillet over medium-high heat for a few minutes until soft. Add duck and cook each side for 2 minutes. Remove pan from heat. Combine remaining ingredients and stir into pan.