This recipe for mustard comes from Le
Menagier de Paris dated around 1393
The recipe is:
Item, et se vous la voulez faire bonne et à loisir, mettez
le senevé tremper par une nuit en bon vinaigre, puis le faites bien
broyer au moulin, et bien petit à petit destremper de vinaigre: et
se vous avez des espices qui soient de remenant de gelée, de claré,
d’ypocras on de saulces, si soient broyées avec, et après la
laissier parer.
Translated that is:
Item: and if you want to make
it good and at leisure, soak the mustard seeds overnight in good
vinegar, then grind it well in the mill, and very little by little
soak in vinegar: and if you have some remnants of spices from jelly,
claré, hypocras or sauces, grind them with it, and then leave to
rest.
I chose a white wine vinegar to soak the mustard in (honestly,
because it is what I had).
The next day, it was interesting to see the difference between soaked and unsoaked mustard seeds.
I then ground the mustard with a mortar and pestle. 
Grinding was a slow process
The final product: a quite spicy mustard
I had to make some
hippocras!
Ypocras. Pour faire pouldre
d’ypocras, prenez un quarteron de très fine canelle triée à la
dent,
et demy quarteron de fleur de canelle fine, une once de gingembre de
mesche trié fin blanc et une once de graine de paradis, un sizain de
noix muguettes et de garingal ensemble, et faites tout battre
ensemble. Et quant vous vouldrez faire l’ypocras, prenez demye once
largement et sur le plus de ceste pouldre et deux quarterons de
succre, et les meslez ensemble, et une quarte de vin à la mesure de
Paris.
Translation:
Hippocras. To make hippocras powder, take a quarteron of very fine cinnamon, sorted by the tooth, and half a quarteron of fine cinnamon flower, an ounce of fine white sorted mesche ginger and an ounce of grains of paradise seed, a sixth of an ounce of nutmeg and galingale together, and beat everything together. And when you want to make the hypocras, take half an ounce and some more of this powder and two quarterons of sugar, and mix them together, and a quarte of wine to the measure of Paris.