Mix flour in a bowl with 1 teaspoon salt. Add eggs and mineral water, mix everything with a wooden spoon with a hole to a smooth dough and beat until the dough is bubbling. The dough should be thick. Let it rest for about 1 hour.
Grate the cheese roughly. Peel onions and slice or cut them into thin rings. Heat butter in a pan. Slowly fry the onions in it until brown.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Bring plenty of salt water to the boil in a large pot. Fill the dough in portions into a button slicer (or a spaetzle press) and slice or press it into the boiling water. Cook for 2-3 minutes until the knöpfle rise to the surface. Remove with a skimmer and drain in a sieve.
Mix the knöpfle with cheese and onions in a casserole dish. Bake in a hot oven for about 10 minutes until the cheese has melted. Wash parsley, shake dry, pluck leaves and chop finely. Sprinkle over buttons.