The onions for an onion soup need a long, slow cooking in butter and oil, then a long, slow simmering in stock for them to develop the deep, rich flavor which characterizes a perfect brew. You should therefore count on 2½ hours at least from start to finish. Though the preliminary cooking in butter requires some watching, the actual simmering can proceed almost unattended.
Cook the onions slowly with the butter and oil in the covered saucepan for 15 minutes.
Uncover, raise heat to moderate, and stir in the salt and sugar. Cook for 30 to 40 minutes stirring frequently, until the onions have turned an even, deep, golden brown.
Sprinkle in the flour and stir for 3 minutes.
Off heat, blend in the boiling liquid. Add the wine, and season to taste. Simmer partially covered for 30 to 40 minutes or more, skimming occasionally. Correct seasoning.
(*) Set aside uncovered until ready to serve. Then reheat to the simmer.
Just before serving, stir in the cognac. Pour into a soup tureen or soup cups over the rounds of bread, and pass the cheese separately.
GARNISHINGS FOR ONION SOUP: Croûtes—hard-toasted French bread 12 to 16 slices of French bread cut ¾ to 1 inch thick Place the bread in one layer in a roasting pan and bake in a preheated 325-degree oven for about half an hour, until it is thoroughly dried out and lightly browned. Olive oil or beef drippings A cut clove of garlic Halfway through the baking, each side may be basted with a teaspoon of olive oil or beef drippings; and after baking, each piece may be rubbed with cut garlic. Croûtes au Fromage—cheese croûtes Grated Swiss or Parmesan cheese Olive oil or beef drippings Spread one side of each croûte with grated cheese and sprinkle with drops of olive oil or beef drippings. Brown under a hot broiler before serving.