For the liver spaetzle, clean the liver and put it through the meat machine together with the onion. Mix the flour with the eggs, then add the liver puree and work both into a smooth dough.
Season with salt, marjoram and parsley. Let rest for about 1 hour. Bring a large pot of salted water to a boil.
Press the liver spaetzle through a coarse-holed sieve or a spaetzle press into the boiling water and cook very gently in it for a good 10 minutes.
Lift out with a slotted spoon and drain well. Heat the butter in a pan, toast the breadcrumbs in it and then spread over the liver spaetzle.