In a hot skillet, add butter and onions. Caramelize the onions. Deglaze with 4 ounces white wine and reduce. Add the chicken stock, beef stock, thyme, and bay leaf. Bring to a boil and simmer for 30 minutes. Adjust taste if needed. Pour soup into bowl and cover with the toasted bread and cheese. Place under broiler until cheese is melted and golden brown.