Season the pork tenderloin with curry powder and let stand for 15 minutes.
In a large, heavy skillet, heat the olive oil over medium-high heat. Add the tenderloin and cook, turning once, until browned on both sides, about 5 to 10 minutes. Remove the meat from the skillet and set aside.
Add the onions to skillet and saute until soft and golden. Add 1 1/2 cups of the apple cider, reduce the heat and simmer until the liquid is half the volume.
Add the chopped apple, cornstarch and the remaining 1/2 cup apple cider. Stir and simmer while the sauce thickens, about 2 minutes. Return the tenderloin to the skillet and simmer for the final 5 minutes.
To serve, arrange tenderloin on a serving platter or divide onto individual plates. Pour thickened sauce over meat and serve immediately.