1. In a Dutch oven, or large pot, sauté the onions with the butter over medium heat, until the onions are soft and golden, about 20 to 30 minutes, stirring often. Add a few drops of water if they get too dry. You want them caramelized but not browned.
2. Add flour, mixing well another 2-3 minutes.
3. Add thyme, bay leaf, sherry, wine and brandy, and simmer uncovered for 10 more minutes.
4. Add the beef broth, salt & pepper to taste, reduce heat.
5. Cover and simmer gently for about 20 minutes.
6. Remove bay leaf and ladle 1-1/2 cups soup into 6 oven-proof bowls.
7. Place bowls on a baking sheet.
8. Place sliced toasted bread rounds into each bowl and top bread with cheese.
9. Broil 2 to 3 minutes, until cheese melts.