Place seeds in a coffee mill or mortar and pestle and grind to desired texture. You can choose to just crack the seeds or you can grind them into dust, depending on your preference.
Place ground seeds into a bowl (for hotter mustard) or a saucepan (for mild mustard). Add mustard powder, salt, sugar, and onion powder.
Pour in white wine and let stand to soak for 10 minutes, to form a paste. For mild mustard, place saucepan on medium low heat and stir for 10 minutes until heated through. Heating the seeds lowers the intensity of the mustard's heat.
Mix in vinegar and add water a little at a time to adjust the consistency of the paste, if desired.
Put your mustard in a jar. Use after 24 hours, when the mustard's flavour has developed. Freshly made mustard will be bitter. This resting time lets that bitterness subside. After 24 hours, you can taste your mustard and adjust the flavour by adding salt, sugar or vinegar, if you like.