Season duck breasts with salt and pepper. Combine next five ingredients in a non-reactive bowl and add seasoned duck breasts. Refrigerate for 1 to 2 hours. Remove duck breasts. Reserve marinade. Heat oil in a large skillet and lightly brown duck breasts on both sides, but not past rare. Remove duck breasts from pan and set aside. To reserved marinade, add apple, mushrooms and brandy. Stir to mix well. Add to pan and cook over medium-high heat until liquid is reduced to about 1/4 cup. Return duck breasts to pan and cook to medium-rare. Remove duck, slice and arrange on plates. Whisk butter into pan until melted and then spoon contents of pan over sliced duck.