In a medium bowl, combine the vinegar, water, yellow mustard seeds, and brown mustard seeds. Cover with plastic wrap and let stand at room temperature for at least 8 hours or up to 24 hours. (The resting time softens the seeds and ensures a creamy consistency.)

Take your vinegar and mustard seeds mixture and measure out 1/2 cup (118 ml) and keep separate.

In a small food processor or with an immersion blender, process the remaining vinegar and mustard seed mixture, sugar, and salt until coarsely ground and thickened, 1 to 2 minutes, scraping down the bowl as needed.

Stir in the reserved vinegar and mustard seed mixture. Using a funnel and spoon, portion mustard into two 1 cup (237 ml) jars. Cover and let the mustard stand at room temperature for 1 to 2 days to allow the ingredients to meld and develop a well-rounded spiciness. The longer it rests, the spicier it gets.

Taste it often and, once the desired spiciness has been reached, refrigerate and serve. (The mustard can be refrigerated for up to 6 months; once refrigerated, the flavor will continue to mature but will not become spicier.)