Add the stock, capers and chopped rosemary to the skillet and boil until reduced to 3/4 cup, about 5 minutes. Return the pork chops to the pan and simmer until cooked through, about 3 minutes. Transfer the pork chops to 4 plates. Whisk the mustard and butter into the sauce and season with salt and pepper. Pour the mustard-caper sauce over the chops and serve.