Weigh the empty jar and set the number aside
Place mustard seeds in the jar and cover with water using twice as much water as seeds, filling no more than ¾ of the jar
Weigh the total of mustard seeds plus water and add 3% salt by total weight
Close the jar and leave it in a cool, shaded place to ferment
Each day, open the jar slightly to check that everything is fermenting properly
Ferment for at least 21 days before use
For mustard spread, blend fermented mustard seeds with apple cider vinegar, honey, and olive oil until smooth or coarse as desired
Add salt to taste if needed
Transfer to the fridge and enjoy
