Mix flour, eggs, egg yolk, 1⁄2 tsp. salt, milk and mineral water to a tough dough. Let the spaetzle dough swell for about 30 minutes.
Bring plenty of salt water to the boil in a pot. Press dough in portions through a spaetzle press into the boiling water. Let the spaetzle simmer until they rise to the surface. Lift them out with a skimmer, let them drain and put them on an oiled baking tray.
Process the remaining dough in the same way.
Roughly chop the nuts. Roast them in a pan without fat and take them out. Heat the butter in the pan. Sauté the spaetzle in it in portions for 3-4 minutes, season with a little salt. Mix in the hazelnuts.