Peel onions, and slice each into 4 slices. Place onion slices in a single layer in a 13- x 9- x 2-inch baking dish coated with cooking spray. Cover and microwave at HIGH 5 minutes.
While onion cooks, combine chutney and pineapple; stir well. Set aside.
Remove onion slices from dish, and place in a single layer on 2 large pieces of aluminum foil coated with cooking spray. Brush onion slices evenly with half of vinegar; wrap tightly in aluminum foil.
Place foil packets, seam sides down, on grill rack over medium-hot coals (350° to 400°). Grill, covered, 10 minutes; turn packets. Open packets, and brush onion slices evenly with remaining vinegar. Top each slice with 1 tablespoon chutney mixture. Loosely wrap slices; grill 5 additional minutes or until onion is glazed and crisp-tender.