Get the biggest Vidalias you can find. Peel and cut a thin slice off the top and bottom of the onion so that it sits flat in a casserole dish. Take out a little of the core and cut a bit X in the top of the onion. Place each onion on a large square of aluminum foil.
Top with pat of butter, salt, pepper and Worcestershire sauce, to taste. Place in dish and bake at 400 degrees for 45 minutes to an hour, or until tender (check on these; you don't want it overcooked and too soft).
Variation: Prepare onions as above, but instead of Worcestershire sauce, pour on about 2 Tablespoons of Kahlua before baking. It makes them nice and sweet. You can also put these on a gas grill or nestle them right down into the charcoal (use heavy-duty aluminum foil).