From Child, Julia, The Way to Cook (New York: Alfred A. Knopf, 1989)

Timing
For the most delicious results, you want a slow simmer of 2 3/4 to 3 hours.

For about 2 quarts, serving 6
3 Tbs butter
1 Tbs light olive oil or fresh peanut oil
8 cups thinly sliced onions (2 1/2 pounds)
1/2 tsp each salt and sugar (sugar helps the onions to brown)
2 Tbs flour
2 1/2 quarts homemade beef stock, at least two cups of which should be hot
4 to 5 Tbs Cognac, Armagnac, or other good brandy
1 cup dry white French vermouth

Special equipment suggested
A food processor with slicing blade or a hand slicer is useful for the onions; a heavy-bottomed 3-quart saucepan with cover for onion cooking and simmering.

Browning the onions—40 minutes
Set the saucepan over moderate heat with the butter and oil; when the butter has melted, stir in the onions, cover the pan, and cook slowly until tender and translucent, about 10 minutes. Blend in the salt and sugar, raise heat to moderately high, and let the onions brown, stirring frequently until they are a dark walnut color, 25 to 30 minutes.

Simmering the soup
Sprinkle in the flour and cook slowly, stirring, for another 3 to 4 minutes. Remove from heat, let cook a moment, then whisk in 2 cups of hot stock. When well blended, bring to the simmer, adding the rest of the stock, the Cognac or brandy, and the vermouth. Cover loosely, and simmer very slowly 1 1/2 hours, adding a little water if the liquid reduces too much. Correct seasoning.

Ahead-of-time note
May be prepared in advance; chill uncovered, then cover and refrigerate or freeze.

Serving
Serve the soup as it is, accompanying it with French bread and a bowl of grated Swiss or Parmesan cheese, gratiné it as follows. d 30 minutes.