In a small (non-metallic) container, jar or glass tumbler, add the mustard seeds and cover with the vinegar.
Leave to stand for 3 days – do not refrigerate. To prevent the kitchen from smelling of vinegar, cover with cling film.
Check periodically – the seeds will swell as they absorb the liquid. Add additional liquid to prevent drying out, it does not matter if you add too much.
After 3 days drain off the surplus liquid and reserve.
Use a hand blender, food processor or pestle & mortar to grind the seeds into a paste. Add the salt, sugar and continue to blend. Add a little extra reserved liquid to get a mustard with a good consistency.
Bottle into sterilized Kilner clip top jars and if possible leave to mature for a few days before using. It should keep for several months and does not need refrigerating.
Recipe courtesy of Kilner