After a turn on the grill, these onions resemble savory roses. They provide an attractive side dish to any grilled meat or fish.
Place each onion half, cut side up, on a 10-inch square of heavy-duty aluminum foil. Drizzle with olive oil and vinegar. Sprinkle with salt and pepper. Turn onions cut side down; pat a piece of bacon onto the side of each one. Fold foil into neat packets.
Heat grill to medium hot. Place foil packets on grill; cook 20 to 30 minutes (if grill becomes too hot, move onions to a cooler area), until tender and slightly charred. Let onions cool in packets for 15 minutes.
Remove foil, reserving juices; discard bacon. Serve onions warm or at room temperature; drizzle reserved juices over the tops.