Sprinkle pork with salt and pepper. In a large ovenproof skillet, heat oil over high heat. Add pork; cook until browned, 3 to 4 minutes per side.
In a medium bowl, combine pineapple, pineapple preserves, vinegar, and mustard. Top pork with half of pineapple mixture.
Bake until a meat thermometer inserted in thickest portion registers 145°, 20 to 25 minutes. Transfer pork to a serving platter, reserving drippings in skillet. Cover pork loosely with foil, and let stand for 10 minutes.
Add remaining pineapple mixture to skillet; cook over medium-high heat, stirring constantly, until slightly thickened, 3 to 5 minutes. Remove from heat; stir in thyme. Pour over pork.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/pineapple-glazed-pork-tenderloin-recipe/