A maple syrup-balsamic vinegar glaze is coated onto this pork tenderloin.
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.