Pork Tenderloin with Roasted Apples and Onions

Place one oven rack in top third of oven, and another rack in bottom third. Preheat oven to 450 degrees. Make glaze: In a small saucepan, bring vinegar and maple syrup to a boil over high heat; cook, stirring occasionally, until mixture has reduced to 1/4 cup, 3 to 4 minutes. Remove from heat; transfer 1 tablespoon to a small bowl for drizzling, and set aside. Reserve the rest of the glaze in saucepan. On a large rimmed baking sheet, toss apples and onion with oil. Season with salt and pepper; arrange in a single layer, and roast until golden, about 15 minutes. Remove from oven and toss. Meanwhile, line another rimmed baking sheet with aluminum foil; place pork on foil. Season generously with salt and pepper; brush with glaze from saucepan. Return apples and onion to oven, on bottom rack. Place pork on top rack; roast 10 minutes. Remove pork from oven; brush with glaze (discard any remaining in pan). Roast until pork registers 150 degrees. on an instant-read thermometer, and apples and onion are tender, about 10 minutes more. Transfer pork to a cutting board, and let rest 10 minutes. Slice 1/4 inch thick; drizzle with reserved tablespoon glaze. Serve with apples, onion, and, if desired, Buttermilk Mashed Potatoes.