Mix flour, eggs, a teaspoon of salt, nutmeg and mineral water to a smooth, thick dough. Let the dough rest for one to two hours. In the meantime, grate cheese finely.
Wash the chives and cut into fine rings. Bring plenty of salt water and oil to the boil. Gradually put a part of the spaetzle dough on the moistened "spaetzle board" and scrape fine strips into the boiling salted water with a long knife.
Bring to the boil until the spätzle rise. Remove the spaetzle with a skimmer and rinse briefly in a sieve under cold water. Drain and keep warm. Process the remaining dough in the same way.
Mix the spaetzle with the cheese and place in an ovenproof dish and bake in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) until the cheese has melted. Peel onions and cut into thin rings.
Heat butter, fry onions in it until golden brown and pour over the cheese noodles. Sprinkle with chives.