Thinly slice the onions and break apart into rings.
In a large soup pan over low heat, melt the butter, the onions and the sugar and mix well.
Sauté the onions very gently, stirring until they are an even golden brown. Add the cognac, raise the heat and flambé. When the flame subsides, lower the heat, sprinkle on the flour and the thyme and mix well. Add the beef stock, raise the heat to high and stir constantly until the soup begins to boil. Lower the heat to medium and simmer for 20 minutes.
Season to taste with salt and pepper.
Before serving, add the soup to individual soup tureens containing a round of toasted French bread topped with a mound of grated Swiss cheese. Put the tureens under the broiler, or in a 400° F oven, until the cheese browns. Serve immediately.Makes 8 Source: