This is an excerpt from Le Viandier de Taillevent
(France, ca. 1380 - James Prescott, trans.)
The original source can be found at
James Prescott's website
Mustard [Sauce]. Soak the mustard seed overnight in good vinegar, grind it in a mill, and then moisten it little by little with vinegar. If you have any spices left over from Hippocras or sauces, grind them with it. (Edited from Power, p. 286.)
Other versions of this recipe:
Mustard (Libre del Coch)
A mustard (Das Kuchbuch der Sabina Welserin)
To make the mustard for dried cod (Das Kuchbuch der Sabina Welserin)
MUSTARD (Le Menagier de Paris)
Mustard; it is fitting to avoid old mustard seed (An Anonymous Andalusian Cookbook)
For lumbardus mustard (Liber cure cocorum [Sloane MS 1986])
Lumbard mustard (Fourme of Curye [Rylands MS 7])