Melt the butter in a heavy-bottomed pot and sauté the onions over medium-low heat until they soften and turn golden brown (keep stirring and adjust the heat so they don't burn). Add the beef stock and simmer 10 minutes. Whisk the grated cheese into the soup and season with salt and pepper to taste.
To serve French Onion Soup Au Gratin, pour into ovenproof soup crocks. Top with toasted, sliced French bread and sliced or grated cheese (Gruyere, Mozzarella, Swiss or Parmesan, or any combination thereof). Broil until the cheese melts and carefully remove from oven.Makes 6 Source: