Melt butter over medium heat. Add onions; cook over medium-low heat 45 minutes or until golden brown, stirring occasionally. Add 3 tablespoons soy sauce and sugar; cook, stirring, 3 minutes. Add stock, sherry, 2 tablespoons soy sauce, thyme and bay leaves. Cover and simmer 45 minutes. Spoon into bowls, top with toasted French bread and Gruyère cheese. Broil until cheese is melted. (Yield: about 3 1/2 quarts)