For the spaetzle, beat flour with salt and two, if necessary three eggs. This also takes 10-15 minutes in the food processor. It must be a very viscous dough. If necessary add some water. Then let it rest for about 10 minutes.
At the same time, put on a really large saucepan with lightly salted water. The water should be just before boiling.
Then, depending on your ability, scrape the spaetzle off the board or use the spaetzle press to add portions to the water. As soon as the spaetzle swim on top, lift them out of the water and store them in a bowl with warm water.
Mix the sauerkraut with the spaetzle and the Kassel pieces. Possibly heat up again briefly.
Cut the onion into rings, fry until golden brown and finally pour over.