(adapted from a recipe in Food and Drink in Medieval Poland)
In a mortar and pestle, grind together: 1 spoonful yellow mustard seed, 1 spoonful black mustard seed; mix with 2 spoonfuls ground yellow mustard.
In a bowl combine mustard powder with 1/2 c. red wine vinegar, 3 spoonfuls olive oil, 1/4 cup honey, and grated pepper to taste.
Grind together and add 1/2 spoonful cubebs, pinch aniseseed.
Add more vinegar or water if needed to produce a runny paste, with a sharp smell as of horseradish.
Macerate 2 cups of parsley leaves in the mortar and pestle, and mix in with the mustard to produce a green paste.
Serve with meat, bread, etc.
This is adapted from the re-created recipes by William Woys Weaver at the back of Food and Drink in Medieval Poland.
The re-created recipe called for Dijon prepared mustard, but clearly that is cheating so I had to come up with a period approximation.
Sarah Garland, in _The Complete Book of Herbs and Spices_ says "The way to reduce mustard seed to fine flour was only discovered in the mind-18th century; before that the seed was pounded as needed in a mustard quern,or the pounded seed was mixed with honey, vinegar and spices and formed into balls that could be stored until needed. John Evelyn's instructions in the _Discourse of Sallets_ written in 1699, are: 'Take the mustard seed, and grind one and half pints of it with honey and Spanish oil and make it into a liquid sauce with vinegar.'" [quoted by Clarkson in Green Enchantment). J.O. Swahn gives instruction for making mustard that basically amounts to grinding your mustard, then adding liquid (wine, water, vinegar, oil, etc.), sweetener if desired, and any other spices.
Green mustard also appears occasionally as a sauce in English cook-books.
Food and Drink in Medieval Poland, Maria Dembinska, revised and adapted by William Woys Weaver. (Philadelphia: University of Pennsylvania, 1999).
"Medieval Mustard", by Terence Graydragon: http://www.greydragon.org/library/mustard.html
The Complete Book of Herbs and Spices, Sarah Garland. (Pleasantville, NY: Reader's Digest, 1993)
Green Enchantment: The golden age of herbs and herbalists, Rosetta Clarkson. (New York, Macmillan, 1940)
The Lore of Spices, J.O. Swahn. (NY: Cresent Books, 1991)