Rub crushed peppercorns evenly over both sides of duck breasts. Season with salt. Brush oil evenly over a medium cast-iron or other ovenproof skillet. Heat the pan over medium-high. Add the duck and cook until browned on the bottom, 2 to 2½ minutes. Turn duck over and transfer skillet to oven. Roast until duck reaches desired doneness: 6 minutes for medium, 10 minutes to cook through. Transfer duck to a cutting board and let rest for 5 minutes. Add any juices accumulated on the cutting board to the Port & Dried Cherry Sauce.
Carve duck on the diagonal into thin slices. Fan on warm plates and spoon sauce over and around duck. Serve immediately.
Tip: Crush peppercorns with the bottom of a small heavy saucepan.