Place ribs in an aluminum foil-lined 13- x 9-inch roasting pan. (Cut ribs in half, if necessary, to fit in pan.)
Bake, covered, at 350° for 1 hour.
Stir together onion, marinade, and orange marmalade. Remove ribs from oven, uncover, and top with onion mixture. Bake, uncovered, 1 hour and 20 minutes or until ribs are tender, basting with pan juices every 20 minutes.