Cook the onions slowly with the butter and oil in a heavy-bottomed, 4-quart 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 beef stock. 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.