Cut onions in half, then thinly slice. In a large, deep pot, heat butter over medium-high heat. Add onions and stir to coat. Cover and cook 10 minutes, stirring often. Add sugar and cook over high heat, uncovered, 6 more minutes or until onions are golden. Add wine and cook 1 minute. Add broth and 2 cups water; bring to a boil, then reduce heat to medium; cover and cook 5 minutes.
Heat broiler. Place four 2-cup ovenproof bowls on a baking sheet. Divide soup among the bowls, stirring in pepper or hot sauce; top each with toasted bread. Sprinkle cheese over bread. Broil 3 to 4 minutes or until cheese is melted.