Cut onion into thine circles and fry in the butter. Sift the flour in the bowl, add mineral water, egg, salt and knead (you can use your food processor). Boil large quantities of water, salt. When the water is boiling, form spätzle directly over the boiling water using a spätzle -maker, cook, remove spätzle as soon as they float. Place spätzle into a preheated butter bowl. Grate coarsely cheese, sprinkle spätzle with it. Place fried onions on the top and serve immediately.