1. 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.
2. 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.
3. Heat broiler. Spread bread slices on a baking sheet; toast under broiler for 1 to 2 minutes per side; set aside. Place six ovenproof bowls or crocks on a baking sheet. Divide soup evenly among bowls (about a cup in each).
4. Use a vegetable peeler or cheese plane to thinly slice pieces of cheese. Place slice of toast on each serving of soup, top with cheese (divide evenly among bowls). Heat under broiler for 3 minutes or until cheese is melted and bubbly. Carefully remove from oven and serve warm.