Lina Wachtelborn Meier, The Art of German Cooking and Baking… (1922)
“ Gravies must always be smooth and it is best to strain them through a fine sieve. If you have them ready before the meal, put them into a double boiler. They must neither be too thick nor too thin. No. 1— WHITE GRAVY. Quantity for 6 Persons. 1 oz. butter 1⁄4 small onion 2 tbsps. of flour 3⁄4 pt. of bouillon 1 pinch of salt and pepper 2 yolks of eggs Preparation: Melt the butter, add the peeled onion and steep a little while, then stir in the flour and bouillon and cook 10 minutes, season with salt and pepper. Beat the 2 yolks of eggs with a little water and slowly stir it into the gravy. ”
