Oh, this is absolutely one of the easiest recipes I know.
Two ingredients : Dry Mustard and Water
Remember a little goes a long way.
Take a couple teaspoons of powdered yellow mustard in a shallow bowl. Add water about 1/4 tsp tsp at a time. The "secret" is in the stirring! Stir with the bottom of a spoon sort of macerating it as you blend the water in. The more you stir the paste the hotter it becomes. Be careful not to add too much water.
That's it!