Preheat oven to 400°. Sprinkle pork with salt and pepper. Cook in hot oil in a large ovenproof skillet over high heat 3 to 4 minutes on each side or until browned. Add onion and plums. Bake 15 minutes or until a meat thermometer inserted into thickest portion registers 145°.
Transfer pork, onion, and plums to a serving platter, reserving drippings in skillet. Cover pork loosely with aluminum foil, and let stand 10 minutes.
Meanwhile, stir vinegar and preserves into drippings; cook over medium-high heat, stirring constantly, 3 to 5 minutes or until slightly thickened. Remove from heat; stir in thyme. Pour over pork and plum mixture.