Cook onions with butter and oil in large covered pot 15 minutes. Uncover, raise heat to moderate, stir in salt and sugar. Cook 30-40 minutes stirring frequently till onions are even deep golden brown. Sprinkle in flour and stir 3 minutes.
Off heat, beat in boiling stock. Add wine, season to taste with salt and pepper. Simmer partially covered 30-40 minutes or more.
When ready to serve, heat to simmer, remove from heat and stir in cognac. Ladle over croutes and pass the cheese.