Cook the onions slowly with the butter and oil in a heavy-bottomed, 4-quart, covered saucepan for 15 minutes. Uncover, raise beat 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.

Reserve a jar of beef bouillon and combine with 3 tbsp flour. Bring the rest of the bouillon to a boil in a soup pot. Off heat, add the jar of combined flour and beef bouillon the wine, and season to taste. Add the browned onions. Simmer partially covered for 40 minutes or more, skimming occasionally.

Just before serving, stir in the cognac. Pour into a soup tureen or soup cups add the bread, and cover with the cheese and place under a broiler until cheese has melted.