In a large kettle, cook the onions in the butter over moderate heat, stirring until they are golden brown. Sprinkle with flour and cook the mixture, stirring, for 3 minutes. Add the broth slowly, stir the soup constantly until it comes to a boil. Simmer, covered, for 20 minutes. Season with salt and pepper. Put 2 slices of toast in each of 6 heated bowls, top each toast with 1 Tablespoon cheese and pour the soup over the toasts. Top with any remaining cheese. Run bowls under broiler about 10 cm from the flame for 3 minutes or until cheese is melted and bubbly. a 1951 Gourmet Mag. favorite.