Rub garlic over surface of trimmed pork loin. Heat a large skillet coated with cooking spray over medium-high heat. Add pork and onion; cook 10 minutes or until browned on all sides, turning occasionally.
Place the pork in a 13- x 9-inch baking dish coated with cooking spray. Sprinkle the sugar mixture over roast. Arrange the onion around pork, and drizzle with vinegar. Bake at 425° for 20 minutes.
Melt butter in a large skillet over medium-high heat; add apples, and sauté 5 minutes or until golden.
Remove the pork from oven, and arrange apples around pork; reduce heat to 350°, and cook an additional 20 minutes or until pork is done.
Pour pan drippings into a medium saucepan, and cook over medium-high heat until reduced to 1/3 cup (about 14 minutes). Drizzle over pork, apples, and onion.