Liberally season duck breasts with salt and pepper. Heat oil and butter in a large skillet over medium-high heat. Add breasts, skin side down and cook until seared to medium brown, about 3 minutes. Flip breasts over and cook other side for 2 minutes more. Remove breasts and transfer to a plate lined with paper towels. Add garlic, onion and brown sugar to the pan. Stir to blend and cook while stirring for 3 to 4 minutes. Stir in beef broth and reduce liquid by one-half. Add bourbon very carefully (see CAUTION above) and cook for 2 minutes more. Add cream and cook until sauce is thickened. Return duck breasts to the pan to warm, but do not cook past medium-rare. Remove breasts, slice diagonally into 1/4-inch slices and spoon sauce over.