In a non-reactive (ie stainless steel or enameled, not aluminum or copper) pot, whisk together the water, mustard, salt, tumeric, and garlic powder. Put over low heat, and mixing frequently, reduce to thick paste. About 40 min.
Mix in vinegar, then continuing to reduce until desired texture. It will only firm up minimally when cold, so reduce until its desired thickness.
Transfer to a clean, air tight jar. I like to use canning jars, but anything similar will work. Let cool on counter until room temperature, then off to the fridge. If you can resist, wait a week before using.