Season the pork with salt and pepper. In an ovenproof fry pan over medium-high heat, warm the oil. Add the pork medallions and cook, turning once, until browned, 4 to 5 minutes. Transfer to a plate.
Add the cider, rosemary and mustard to the pan, bring to a boil and stir to scrape up any browned bits on the pan bottom. Cook until the liquid is reduced by half, 3 to 4 minutes. Return the pork to the pan, place in the oven and cook for about 6 minutes. Remove from the oven, turn the pork and add the sliced nectarines. Return to the oven and cook until the pork is tender and registers 145°F (63°C) on an instant-read thermometer, 6 to 8 minutes.
Transfer the pork and nectarines to a platter. Place the pan over medium heat and whisk in the butter to make a sauce. Spoon the sauce over the pork and nectarines and serve immediately. Serves 4.
Apples are a perfect substitute during fall and winter.