Slice the tops off of the onions, and remove the outer layer of skin. Leave the base intact. Cut slices across the top in a cross hatch pattern, going about 1/2 inch into the onion. Set each onion on a square of heavy duty aluminum foil. Double the foil if you do not have the heavy duty. Place 1 tablespoon of butter on top of each onion, then place one bouillon cube onto the butter. Fold the foil up around the onion. When almost closed, place 3 ice cubes on top of the onion, and spoon 1 tablespoon of the sherry into each packet. Seal completely.