1. In a 3- to 4-quart pan over medium-high heat, stir onions and balsamic vinegar often until liquid is evaporated, 1 to 2 minutes.
2. Add broth, thyme, and bay leaf; bring to a boil. Reduce heat and simmer, stirring occasionally to blend flavors, 10 minutes. Add salt and pepper to taste.
3. Meanwhile, place baguette slices in a single layer on a 12- by 15-inch baking sheet. Broil 4 to 6 inches from heat until lightly browned on top, 1 to 2 minutes. Turn slices over and sprinkle evenly with cheese. Broil until cheese is melted, 1 to 2 minutes longer.
4. Remove thyme and bay leaf; ladle soup into bowls. Serve with toasts.