This is a recipe from my (German) daughter-in-law. I'm giving it to you exactly as she makes it because someday you might want to make the authentic dish. But I just made it with cauliflower and double mustard. It was scrumptious. Also good enough for company, and economical.
"This for 4 portions - as you know most of the things taste better once they have been sitting for a day;-)
200 gram potatoes
8 eggs
150 grams low fat milk
2 teaspoons mustard (or more!) - you can try 1 spoon each of 2 different kinds if you like
onion, instant-vegetable stock, salt, pepper, sugar
peel potatoes and the onion, chop both is cubes and boil for 15 minutes in 250 grams of vegetable stock. at the same put the eggs in boiling water and boil them for 8 minutes, than shock and peel. add the milk to the vegetable stock and the potatoes and puree all. than add the mustard to the sauce and add salt, pepper and some sugar. normally one would have boiled salted potatoes to go with that. I like it since the base of the sauce is pureed potatoes rather than roux."
The Prater is a old Berlin institution, restaurant and summer beer garden -- where I first had this dish.
http://www.pratergarten.de/e/popups/gaststaette/01.html