Peel onions and cut them into thin rings with a sharp knife or slice them into fine rings on a vegetable slicer. Heat the oil in a large pot. Fry the onions for about 10 minutes at low heat until they are glassy.
Sprinkle flour over the onions and sauté lightly. Stir in wine and ¾ l water bit by bit, add broth. Season with salt and pepper. Bring to the boil and simmer covered for about 20 minutes.
Preheat oven: Electric cooker: 225 °C/circulating air: 200 °C/gas: level 4 Cut the baguette into thin slices and roast without fat in a pan or toaster.
Finely grate the cheese. Wash parsley, shake dry and chop finely. Season soup with salt and pepper and refine with cognac. Pour half of the soup into ovenproof soup bowls or plates.
Place 1 slice of baguette on each and pour some of the soup over the slices. Sprinkle with cheese and bake in the oven for 5-7 minutes. Sprinkle the soup with parsley.