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 the flour and stir for three
Off heat, blend in the boiling liquid.
Add the wine and season to taste. Simmer partially
covered for 30 to 40 minutes of more, skimming occasionally.
Pour into a soup tureen or soup cups.
Float the rounds of toast on top of the soup, and spread the
grated cheese over it. Sprinkle with the oil or
butter. Bake for 20 minutes in the oven, then set for
a minute or two under a preheated broiler to brown the top
lightly. Serve immediately.
Just before serving, stir in the cognac.