Peel and wash the onions. With the point of a small, sharp knife, cut a 1-inch core from the top of each onion and make shallow slits in a circle around the top. Wrap 2 slices of bacon horizontally around each onion, and secure them with toothpicks. Then put 1 teaspoon of butter in each core. Season with salt and pepper.

Place each onion on a square of aluminum foil and bring the edges together at the top, leaving a small opening for steam to escape. Put the foil-wrapped onions on the grill and cook for 1 hour, or until the onions are tender when pierced with the tip of a knife. You can also bake in the oven at 350 degrees F for 1 hour. Cool the onions for a few minutes then unwrap and cut into quarters to serve.