Pre-heat oven to 425 degrees F.Lightly spray roasting pan with cooking spray.In a large skillet, heat olive oil and add the pork tenderloin and sprinkle with pepper.Cook the tenderloin until it is browned on all sides, about 3 minutes.Remove from skillet and place in roasting pan.Roast the pork until it registers 160 degrees F with a meat thermometer, approximately 20 minutes.
Meanwhile, in a skillet, melt 1 tablespoon of unsalted butter and then add the apple, onion and rosemary.Sauté until the onion is translucent and the apples are soft, about 3-5 minutes.Add the chicken stock and balsamic vinegar and bring mixture to a low boil.Continue boiling until the sauce has reduced, approximately 5 minutes.
To serve, place pork tenderloin on a platter and pour apple-onion mixture over the top of the tenderloin.Serve immediately.