Melt the butter in a heavy-bottomed pot and sauté onions over medium low heat
until they soften and turn golden brown. Keep stirring so they don't burn. Add
beef stock and simmer for 10 minutes. Whisk grated cheese into soup and season
with salt and pepper to taste.
Pour into ovenproof soup crocks. Top with toasted, sliced French bread and
grated cheese (Gruyere, Mozzarella, Swiss or Parmesan). Broil until cheese melts
and remove from oven.