For the mustard seeds, take some whole mustard seeds put them in fresh water, bring them to a boil, drain and rinse them to purge their bitterness. Repeat this process to reduce bitterness.
If desired, dice up some aromatic vegetables like red chili and shallot. Combine the drained seeds with vinegar and water, if desired. Add sweetener, salt, and ensure the seeds are submerged with extra liquid on top. Boil for 7-8 minutes.
Add diced vegetables and boil for a couple more minutes until the seeds are plump. Adjust vinegar, water, salt, sugar to taste. Eat hot or chill in the fridge for improved flavor. Can be stored in the fridge for at least a couple of weeks.