1. Season the pork tenderloin with curry powder and let stand for 15 minutes.
2. 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 (5-10 minutes). Set meat aside.
3. 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 halved (20-25 minutes).
4. Add apple, cornstarch, and remaining 1/2 cup apple cider. Stir and simmer while the sauce thickens (2-3 minutes). Return the tenderloin to the skillet and simmer until pork reaches internal temperature of at least 145 degrees F (5-6 minutes).
5. To serve, arrange tenderloin, apples, and onions on a serving platter or divide onto individual plates. Pour thickened sauce over meat and serve immediately.