Grind the mustard seeds coarsely in a spice grinder, just enough to crack them. Pour them into a pint-sized jar. Then stir in the mustard powder, salt, water and starter culture.
Seal the jar tightly, and then let the mustard ferment for 3 days at room temperature.
Uncover the jar, stir in the vinegar and honey, and then it to the fridge, where it will keep about 6 months.