I have a recipe for mustard that calls for fermented mustard seeds - is there a way to ferment them at home?
Whey appears to be an agent for lacto-fermenting mustard. This recipe for lacto-fermented mustard describes a process of combining ingredients and then letting them ferment after the recipe is put together by leaving out over a few days.
This recipe for a mustard seed raita uses yogurt as the fermentation ingredient, which means that you would have to use yogurt with active cultures.
There's a reasonably involved text behind the link on home fermentation. Not specific enough an answer for you, sorry. My google-fu was not strong enough for this one. It may help you get started though.