Roman mustard recipe based on Columella (De Re Rustica 12:57:2).
For a more authentic taste use wine vinegar from Spain, either Sherry or Sweet Moscatel, which have a stronger taste, or Sweet Pedro Ximenez vinegar, which has a milder taste. All of the above mentioned vinegars are made based on recipes closer to what the Romans would have used. A very good brand of Spanish wine vinegars is Los Villares, which can be purchased in Whole Foods Supermarkets throughout the US.