In a medium saucepan, combine 1/2 cup of the sugar with the wine and bring to a boil, stirring to dissolve the sugar. Add the red onion, cover and cook over high heat, stirring occasionally, until softened, about 5 minutes. Drain the onion slices and discard the liquid. Return the onion to the pan. Add the remaining 1/2 cup of sugar and the red wine vinegar and cook over moderate heat, stirring occasionally, until syrupy, about 6 minutes longer.