To prepare soup, melt butter in a medium saucepan over medium heat. Add onion, sugar, and pepper to pan. Cook 15 minutes or until golden brown, stirring occasionally. Stir in wine; cook 2 minutes or until wine almost evaporates. Add broth, 1 1/2 cups water, and thyme; bring to a boil. Cook until reduced to 2 1/2 cups (about 15 minutes). Place 1 1/4 cups soup into each of 2 ovenproof soup bowls. Place bowls on a baking sheet.
Top each serving with 1 slice baguette. Sprinkle 1 tablespoon cheese over each serving. Broil for 3 minutes or until cheese melts. Serve immediately.