There are few things more pitiful in this world than the fall of mustard into its current vapid form. Mustard seeds were but a simple spice until around 2000 years ago, when the Romans brought civilization to the world -- including mustard. As the name clearly states, the Romans didn't actually see the mustard seed (in Latin, sinapis) as central to the dish. They called it mustum, a word used as both a shortening of mustum uinum, or 'new wine', and mustum ardens, or 'burning must'.
Of course, simply adding mustard seed to unfermented crushed grapes would be somewhat underwhelming. The Roman cookbook Apicius suggested using mustum ardens as a glaze for whole roasted boar, and called for a heady mixture including ground mustard seed, black pepper, caraway, lovage, roasted coriander seeds, dill, celery, thyme, oregano, onion, honey, vinegar, fish sauce, and oil. This fell somewhat short of other recipes that included pine nuts and almonds, which would give the resulting condiment even more body.
Apicius does not give the details on preparation, but if prepared the same day and used as a marinade over a hot spit, it is likely that the heat mellowed out the mustard considerably, and the lack of a sitting period may have resulted in a bitter taste. However, the Romans certainly were aware of the range of mustard's bite, as when Titus Maccius Plautus referred to 'roguish mustard', the overuse of which would liken food to being seasoned with vampire owls. No, I don't know what that means either. However, the word mustum certainly referred to much more than the seed, or a simple paste resulting therefrom.
Although there are a number of recipes for 'Roman Mustard' floating around, I would recommend starting with the fairly simple and representative recipe given by The Splendid Table, which involves soaking the mustard seeds and mixing them with vinegar and grape juice, and allowing the slurry to ripen for around two days. The mixture is completed with cumin, pine nuts and almonds, giving a more hearty result than you are likely to find in modern supermarkets, although perhaps less suitable for marinating roast boar than a centurion would prefer.
Bonus mustard word!
Mustardarius: the monk in charge of planting, harvesting, and preparing mustard.