This is an excerpt from Le Menagier de Paris
(France, 1393 - Janet Hinson, trans.)
The original source can be found at
David Friedman's website
BARLEY GRUEL. Soak the barley in a basin for about half an hour, then mash it and put it in a copper mortar and crush it with a wooden pestle, then set it to dry: and when it is dry store it. And when you want to make broth, put it to cook in a little pot with water, and when it bursts, mash it and put it on to boil with almond-milk; and some strain it.
Item, add lots of sugar.
Other versions of this recipe:
Oatmeal Gruel and Barley Gruel (Libre del Coch)
Hulled barley gruel (Le Viandier de Taillevent)