The vidalias don't have the papery skin to remove. Even out the root end so that the onion will sit flat, trim the stem about 1/4 in below the green tops, reserve some of the green to chop for garnish.
I used the small end of a melon baller to remove a core of the onion and added about 1/8th teaspoon bouillon seasoning in the cavity followed by a little pat of butter and sprinkles of porcini salt and a little pepper, topping with a bit of the chopped green tops.
Wrap each onion in foil and close tightly, grill or roast for 25 minutes. Serve one onion per person as a side to grilled steak.