Mix the sugar, salt, and wine or bourbon in a small bowl until the sugar dissolves. Gently oil each round and place on a medium hot grill. After a minute or so, flip the onions and baste with a teaspoon of the sugar mixture. Be careful — the alcohol can easily flare up. Flip the onions several times and baste twice between flips. The liquid keeps them from drying out and the sugar makes them super sweet.
According to the Vidalia Onion Committee Web site, www.vidaliaonion.org, "So, what is it about Vidalias that makes them so sweet? Southeast Georgia's mild climate, the area's sandy, low sulfur soil, exclusive seed varieties, and precise farming practices make this original sweet onion mild and flavorful."