This is a very traditional Parisienne gratinéed soup — very delicious and warming on a cold winter’s evening. (Please please never serve it with croutons and grated cheese.) Bon Appetit!
Melt the butter in a deep saucepan and add the onions with a sprinkling of salt. Cook 30 minutes over medium high flame. Towards the end of the time, it is important to stir the onions with a wooden spoon to keep them from sticking–they should be a nice dark brown color. Lower the heat and stir in the garlic. Cook 1 minute. While stirring, sprinkle in the flour and mix well. Cook 3 minutes or until the flour is slightly brown. Stir in the white wine, stock, salt, and pepper and simmer for another 30 minutes. Stir in the sherry, taste and adjust the seasoning. (this soup can be stored in the fridge for 5 days or frozen for up to 6 months.) When ready to serve reheat the soup in a saucepan, ladle it into ovenproof soup bowls, dunk two slices of bread into each bowl, sprinkle generously with the cheese, and run under the broiler until golden. Note: The best Gruyere is Cave-Aged from Switzerland (not Wisconsin) and should be grated on the large holes on the grater. Parmesan being a dry cheese should be grated on the small holes on the grater. (Please never buy already grated Parmesan–it simply doesn’t have good enough flavor.)