The key to French Onion soup is the slow cooking of the onions in butter and oil, followed by a long, slow simmering in stock. This helps them develop the rich flavor this soup is known for.
Julia Child's French Onion Soup (Soupe à L'oignon Gratinée)
5 cups thinly sliced yellow onions
3 tablespoons butter
1 tablespoon oil
1 teaspoon salt
1/4 teaspoon sugar
3 tablespoons flour
2 quarts beef stock, boiling
1/2 cup dry white wine
salt and pepper to taste
3 tablespoons cognac
4-6 rounds of hard-toasted French bread
1-2 cups grated Swiss or Parmesan cheese
1) Cook the onions slowly in the butter and oil in a covered saucepan for 15 minutes. Uncover, raise heat to medium and stir in the salt and sugar. The sugar will help the onions to brown. Cook for 30-40 minutes, stirring frequently, until the onions have turned a deep golden brown.
2) Sprinkle in the flour and stir over heat for 3 minutes. Off heat, blend in the stock. Add the wine and season to taste. Simmer partially covered for another 30-40 minutes or more, skimming if needed. Correct seasonings.
3) Just before serving, stir in the cognac. Place rounds of bread in soup bowls or a tureen and pour soup on top. Sprinkle with grated cheese and brown under a hot broiler until golden and bubbly. Serve immediately.