1. Place rack in lower third of oven and heat oven to 375°F. Place ham in a roasting pan and scatter onions around. Add the reserved pineapple juice to the pan. Cover pan snugly with foil and roast 1 hour.
2. In a measuring cup, stir together maple syrup, mustard, and vinegar. Remove pan from oven and carefully remove foil. Brush ham with maple mixture, add pineapple chunks to pan, and roast, uncovered, 30 minutes longer. Transfer ham to a cutting board. Slice and place on a serving platter along with onions and pineapple.