1. In a saute pan, steam fry minced mushrooms in a little vegetable stock until soft.
2. Mix the cornflour in with the remaining vegetable stock and add it, a little at a time, to the pan. Bring to a boil, then remove from heat.
3. Blend with the cashews in a food processor until smooth - this takes a while; be patient.
3. Stir in the prepared mustard last. You don't want to add it any earlier because (Fun Science Fact!) the chemical compounds that give mustard its heat and flavor break down at high temperatures. You can control the spiciness of the sauce by adding the mustard when it's piping hot, or letting it cool down slightly. But cooking the mustard would just make it tasteless.
Serve with vegetables, as a gravy, or (cooled) over salads. Or do what I did and eat it out of the pan with a spoon. Whatever floats your goat.
Source of recipe: Dinking around in the kitchen, improvised this.