Grate cheese. Peel onions and cut into rings. Heat oil in a pot, fry onions in it, dust with flour, sauté, deglaze with broth and wine. Let simmer for about 8 minutes.
Meanwhile roast bread. Cut out 4 circles from each slice of bread and place them on a baking tray lined with baking paper. Sprinkle with cheese and bake in a preheated oven (electric cooker: 200 °C/ gas: level 3) for about 4 minutes.
Wash parsley, dab dry and chop finely. Season soup with salt, pepper and sugar. Fill the soup into four soup bowls, place two slices of bread on top and serve sprinkled with parsley.