Melt butter in a very large, deep pot over medium heat. Add onions and stir to coat. Cover; cook 15 minutes , stirring occasionally, until onions are very soft and begin to turn golden brown.
Uncover; increase heat to medium-high. Add sugar and cook, uncovered, 10 minutes, stirring often. Sprinkle with flour, cook 1 minute. Stir in broth, wine and 1 cup water. Simmer, uncovered for 5 minutes.
Heat broiler. Spread bread slices on a baking sheet. Toast under broiler for 1 to 2 minutes per side. Set aside. Place ovenproof bowls on baking sheet. Divide soup evenly among bowls.
Place slice of toast on each serving of soup. Top with cheese. Heat under broiler for 3 minutes or until cheese is melted. Carefully remove from oven & serve warm.