Place the water, mustard powder, salt, turmeric and garlic in a small sauce pan and whisk until well blended.
Cook over medium to low heat, stirring often, until it turns into a thick paste. This is the part that takessome time, about 30-40 minutes. Make sure all of your windows are open, as the mustard might start to burn your eyes a little.
Add the vinegar to the mustard mixture, and continue to cook until it’s reached your desired consistency; about 5-15 more minutes.
Next, remove the pan from the heat and allow it to cool. Store in an air-tight container in the refrigerator, and wait at least a few days before digging in. The flavor gets better and mellower as time goes on.