This is a staple of BBM. I would guess that there’s a version in Gute Spise, but it’s not like that matters.
Mustard Soup
1 lb butter (unsalted if possible. If not, salted is okay)
1 lbs flour
16 quarts (64 c) veg stock
2 dozen eggs
60 oz of mustard
1 gallon milk
1 gal whipping cream
Heat the chicken broth and milk until it is hot but not boiling (this takes about an hour on a fairly high heat). Make a roux by melting the butter in the bottom of a large pot, gradually adding the flour while stirring constantly until you have a thick paste.Add the chicken broth/milk mixture, stirring CONSTANTLY to prevent lumping. Beat the eggs in a separate bowl. Add the mustard and mix well. Add the cream and mix well. Mix a little of the hot broth into the egg, mustard, and cream mixture to warm the mixture so the eggs don’t curdle, then add the entire mixture to the large pot of broth.
Leaves of Silk and Glass 