Peel and thinly slice the onions. Heat the fat in a pot and fry the onions in it until transparent. Deglaze with white wine and vegetable stock. Let it boil up. Cook on low heat for about 5 minutes. Wash parsley and chop finely except for a few leaves for garnishing.
Finally add to the soup and season with salt and pepper. Briefly toast bread in a preheated oven (electric cooker: 200 °C/ gas: level 3). Grate the cheese and sprinkle it on the bread. Bake for 4-5 minutes. Serve the soup in portions and add 1 slice of gratinated bread. Serve garnished with parsley as desired