In a crock pot, cook onions and margarine on low for 10-12 hours. Do not lift the lid during cooking time, tempting though it may be. After cooking, skim off the margarine/butter.
Add broth, wine, balsamic vinegar, mustard, bay leaves, thyme, and a dash of salt to the crock pot. Cook on high for 1/2 to 2 hours. Add additional salt to taste.
You can refrigerate the soup overnight. Just be sure to reheat the soup to a simmer before doing the next step.
Slice bread and toast in 350F oven for 15 minutes, until bread is dried out. Add hot soup to oven-safe bowls. Add bread slices and top with a slice of vegan cheese (mozzarella or provolone). Broil until cheese melts and starts to brown in spots.