Peel onions and cut or slice them into fine rings. Heat the oil in a large pot. Fry the onion rings in it at low heat for about 10 minutes until transparent. Season with salt and pepper.
Dust the onions with flour and sauté. Stir in wine, 1 l water and stock. Bring everything to the boil, cover and simmer for 15-20 minutes.
Preheat the oven (electric cooker: 220 °C/circulating air: 200 °C/gas: level 4). Cut rolls into 6 slices and roast in the oven for approx. 5 minutes. Grate cheese finely.
Season soup with salt, pepper and cognac. Divide into 6 ovenproof soup bowls. Place 1 slice of bread on each and sprinkle with cheese. Bake in a hot oven for 6-8 minutes until golden brown.