Combine all ingredients in a small saucepan and cook over medium to medium-low heat for 6 minutes. Stir frequently until it thickens and then remove from heat. Allow the mustard to cool before storing it in an airtight container. Depending on how spicy you like your mustard, you can refrigerate it at this point, which will preserve its fiery characteristic, or allow it to sit at room temperature for a day or more which will mellow its flavour the longer it sits out.
Note: You can also play around by adding different herbs, sea salts, ale, wine, chillies, peppercorns, horseradish, etc.
Chef's Note: All you need to hold together a great vinaigrette or marinade is a tablespoon of this mustard. Place mustard in a bowl and slowly whisk in the vinegar of your choice. Whisking quickly and continuously, follow with the olive oil of your choice. The key here is to whisk briskly so that it creates a homogeneous emulsification that hold the oil and vinegar together as opposed to what it would normally do which is separate.