1 cup ground yellow mustard powder
1/3 cup water
1/2 cup Serrano Honey Vinegar
2 teaspoons salt
Grind the whole brown mustard seeds in a spice grinder for 15 seconds or with a mortar and pestle. They should be cracked but not fully ground to powder.
Combine the slightly crushed seeds, mustard powder and salt in a small bowl.
Cover and store at room temperature for 2 days to allow the mustard to thicken and become less harsh. Allowing the mustard to sit at room temperature mellows the bite. The longer you allow it to "age" the more mellow it becomes.
Once the mustard is of the desired pungency, transfer to a clean glass and refrigerate covered tightly.