In a large non-stick pan, cook onions, and butter over medium-low heat until golden and caramelized. (About 20 minutes).
Once golden, add to the slow cooker along with the remaining ingredients except for bread and cheese.
Cook on low for 6 to 8 hours.
Remove and discard bay leaf and ladle the soup into bowls.
Top with dry bread slices and cheeses.
Broil 2 to 3 minutes or until cheese is melted and browned.
