In a 6 quart stock pot, melt butter.
Add onions and sauté until cooked down and starting to brown.
Add wine and cook an additional 5 minutes.
Add chicken stock, beef stock, and thyme.
Simmer for 15-20 minutes.
Ladle into serving bowls.
Top with bread slices and cheese.
Place under broiler until cheese is melted and edges of bread are toasted.
