Melt butter in a Dutch oven over medium-high heat; add onions, and cook, stirring often, 30 to 40 minutes or until golden brown.
Add chicken broth and next 4 ingredients; bring to a boil. Reduce heat, and simmer, stirring occasionally, 20 minutes. Remove and discard herbs. Add salt and pepper to taste.
Ladle into 6 ovenproof bowls; top with bread and cheese slices. Broil, 5 1/2 inches from heat, 3 minutes or until cheese is browned and bubbly.