Grilled Sweet Onions

Preheat an outdoor grill for medium heat. Peel brown outer layers from an onion. Using a sharp knife, cut a cone-shaped core from the top of the onion, about 2/3 of the way into the vegetable. The core should be about 1 1/2 inches wide at the top. Insert a bouillon cube into the hollowed-out space, followed by an ice cube (for more moisture, if desired). Top the ice cube with 1 tablespoon of butter. Wrap the filled onion in a sheet of aluminum foil, leaving a small vent at the top. Repeat with the rest of the onions and filling ingredients. Set the foil-wrapped onions on the preheated grill vent-sides up, and grill until onions are tender, 20 to 30 minutes.