Cut the onion in thick rings and place on the bottom of a big pot.
Cut the gingerbread in pieces and add to the pot.
Cut the lemon in slices and add to the pot.
Cut the parsnip or parsley root in pieces and add to the pot.
Add the vinegar, cloves, salt, sugar, bay leaf, vinegar and all the beer to the pot.
Over medium heat bring to a boil. NOTE: this will boil over worse than milk so stay close to the pot. Turn heat to low and simmer until everything is soft.
Pass the sauce through a strainer and push softer pieces through. The sauce should be thick and smooth.
Return the sauce to the pot and season to taste with salt, sugar, lemon and/or vinegar.
Add the butter.
Gently heat the white sausages in the sauce (do not boil!). Serve over boiled potatoes and sauerkraut with a generous amount of sauce.