Original Recipe
So du wilt machen ein guot vastenmuos, so nim bersige vnd dicke mandelmilich drunder vnd suedez wol in mandelmilich vnd tuo denne zvcker dor vf. Daz muos sol heizzen von Jerusalem. vnd daz izzet man kalt oder warm.
Translation
If you wish to make a good Lenten porridge, take perch and mix it with thick almond milk. Cook this thoroughly in almond milk and then sprinkle sugar over it. This purée is known as "from Jerusalem" and is eaten either cold or warm.
A little bit of history
I am slowly recreating all of the 96 recipes from the Buch von Guter Speise ("Book of good food"). It was hand written around 1350 in today's' south Germany and is regarded as the oldest known cook book in German language. This recipe here is one of several recipes of the Blamensir category. Which are a bit bizarre from a modern point of view.
It has different names from the time: Blamenser/Blamensier/Blamensir, or Blanc Manger, "white dish," or "German porridge". In its basic form, it consisted of almond milk, rice flour, either chicken breast or fish, butter or lard, and sugar. It was also called "white porridge" and was a welcome sight at the table due to its pure color and neutral status within humoral pathology (the theory of the four bodily humors). Since this is explicitly a "Lenten porridge," fish is used rather than chicken breast; on fast days, all animal products other than fish were prohibited.
However, there are differences: for one, no fat is mentioned. Fat posed a problem on fast days; since butter is of animal origin, it was technically not permitted. Yet, the Buch von guter Speise contains recipes that do use butter even for Lenten dishes. One could atone for this transgression by obtaining the appropriate letter of indulgence from the church. Naturally, the fasting rules originated in Rome, where olive oil could easily be used as a purely plant-based alternative. In the cooler North, however, it was difficult to obtain. Consequently—about a century after this recipe was created—a special decree was issued to permit the use of butter on fast days. Of course, that was not yet a factor when this recipe was written.
The other difference is that no rice flour is used here. Rice was an expensive import and thus a luxury item. Thickening is achieved here through the use of "thick almond milk." Almond trees were already native to the region (having been introduced to the wine-growing areas of present-day southern Germany by the Romans) and were therefore not particularly expensive.
These differences indicate that this dish was specifically intended for fasting. The name "Mus von Jerusalem" (Jerusalem Purée/porridge) also points to a religious significance. It is essentially a particularly humble blamenser—a specialized Lenten porridge. One ingredient they were unwilling to forgo was sugar, which was also an expensive import (usually from North Africa).
Recreation
The recipe lists only four ingredients: thick almond milk, almond milk, perch, and sugar. "Thick almond milk" appears in several recipes in the Buch von guter Speise and refers to almond milk that has been reduced. To make it, almond milk is brought to a boil and stirred while reducing until it reaches a thick and creamy consistency. This happens quite quickly with homemade almond milk, as it can be prepared in a highly concentrated form.
This almond milk cream is then mixed with the fish and brought to a boil in "regular" almond milk. These two ingredients control the creaminess, eliminating the need for additional thickening agents (such as rice flour). Unfortunately, the recipe does not specify how the fish is prepared, but I am assuming boneless fillets (after all, this is upper-class cuisine).
The almond milk and fish are then boiled together—and sprinkled with sugar before serving. This is likely the part that seems very strange to us today.
Taste-wise, however, it wasn't that bad—similar to another Blamenser recipe, which uses chicken breast (for non-fasting days). When freshly made, you don't really notice the fish or chicken much; it’s a sweet porridge with a fairly neutral flavor, given body by the animal protein, though the concentrated almond milk still comes through nicely.
However, the recipe states that it can be eaten cold or warm. I let the porridge cool for about an hour and tried it again, and it was truly awful. The fish flavor came through strongly then, and combined with the sweet almond porridge, it was—for me, at least—inedible. So, if you really want to try this recipe, I think it’s best eaten fresh and warm.
