Bring all ingredients, except for the mustard seeds, to boil in a medium stock pot. Simmer covered for 10 minutes. Turn off the heat and let steep for 1 hour. Strain the ingredients and place the vinegar solution in a large non-reactive bowl (preferably glass).
Stir the coarsely ground mustard seeds into the vinegar solution. The mixture will initially be liquidy and will thicken over time. Cover and refrigerate for at least 3 weeks before using to allow the flavors to meld and deepen. The mustard will initially be quite hot but over this resting period the heat will become more subdued.
Stored in the fridge, this mustard will keep up to 6 months.
Note: This recipe can be halved or quartered if you don't want to make a large amount.