Mustard was cultivated and eaten in Rome, and was known in France at the time of Charlemagne, and in England and Germany by the 12th century (1100's). The Christian Bible speaks of one who has 'as much faith as a mustard seed' being able to 'remove mountains'; when you consider that the tiny black mustard seeds grow into 6-foot-high plants sturdy enough for birds to nest in them in a single summer, you can understand why! The yellow mustard plants you see in fields in the spring are a relative of mustard, Brassica Sinapstrum, also called charlock.
Mustard sauces were pretty popular in the middle ages. Rosetta Clarkson, in Green Enchantment: The Golden Age of Herbs and Herbalists, says that some monasteries actually had a monk called the 'mustardarius' whose duties includied mixing the mustard sauce for the community. Mustard sauce could be used on meat or on fish, and in the days when you ate fish three times a week at least, and people ate a lot of cold, pre-roasted meat, no wonder it was popular! Le Menagier de Paris suggests mustard sauce with wild boar, beef tongue, and lots of different fish, including eel, shad, loach, lampreys, cod, stockfish, and whiting.
Mustard sauces (see attached article by Terafan Greydragon) were generally made with ground mustard seeds-- black was considered better than white-- (sometimes mixed with other spices such as pepper), moistened with 'wine must' (fresh red or white grape juice), vinegar or wine. Honey or sugar was also added in a number of recipes; breadcrumbs, almonds and fruit such as raisins appear in some recipes. (Platina says, "If you want it sweet, add sweet things; if sour, sour.")
Parkinson, in his Paradisus in Sole (1629), says "The seede ground between two stones, fitted for the purpose, and called a Querne, with some good vinegar added unto it, to make it liquid and running, is that kind of Mustard that is usually made of all sorts, to serve as sauce both for fish and flesh."
The Forme of Cury says to make Lombard Mustard you wash and dry your mustard seed (to get the chaff off), grind it, sieve it, and mix in clarified honey, vinegar and wine to make a thick paste, and then thin it with wine to serve. (http://www.pbm.com/~lindahl/foc/FoC108_smallgif.html)
Because mustard was is spicy and hot, medieval doctors and health-hobbyists like Platina suggested it to counteract 'cold' foods and 'cold' conditions. It was drunk and gargled with in wine for sore throats; Dioscorides (a first-century Greek) suggested 'mustard plasters' to help with 'pain of long continuance' (probably on the same principle as Tiger Balm.
Nowadays we buy mustard flour, ground in the same manner as wheat flour,
but Sarah Garland in The complete book of herbs and spices, and Rosetta
Clarkson in Magic Gardens: A modern chronicle of herbs and savory seeds,
say that mustard flour was not invented until the 18th century (1700's).
Instead, you could buy mustard meal in some places: Plat's Delights for
Ladies says: "It is usuall in Venice to sell the meal of Mustard in their
markets as we doe flower and meale in England: this meale, by the addition
of vinegar, in two or three daies becommeth exceeding good mustard." (Apparently
he liked his mustard mild too.) But mostly you ground it at home, either
with a mortar & pestle or with a mill in later times. You
could also buy your mustard sauce ready-made, if you lived in the city: the
14th century Le menagier de Paris directs the reader to buy "At the
sauce-maker, a quart of cameline for the dinner, and for supper two quarts
of mustard. "
Note: if you use a coffee grinder to grind mustard seed, be sure to wash the washable parts and clean the electrical parts by wiping it out, grinding dry bread in it and wiping it out again both before and after grinding the mustard.
The 'hotness' of mustard tones down if you keep it for a week or more after mixing the sauce, and it seems that most of the people who wrote the recipes that have survived liked it that way; they stored their mustard for some time before using it.
We will make a simple mustard, mixing it with honey and wine vinegar or
grape juice. Cinnamon, raisins, pepper, sugar and aniseed (aniseed appears
in a mustard in an Icelandic manuscript) are available for you to mix in with
it if you like.