My husband loves really hot mustard, hotter than he has ever been able to find in a store. Fortunately it’s easy to make our own – and wow, it’s really hot!
“Regular Canadian Mustard Powder (medium)” from Penzey’s. Or you may be able to get Colman's Mustard powder at your supermarket, which is almost as hot as Penzey's. For ordinary hot mustard you could reconstitute this with vinegar, wine or water. But that’s not hot enough for Ken, so we reconstitute it with a concoction we call hot vinegar.
Hot vinegar is made not on the stove but by putting hot peppers into a glass jar and covering them with plain white vinegar, then putting them on a dark shelf at room temperature. A couple of weeks (or years) later the vinegar has become infused with lots of pepper heat. After you pour off some for a batch of mustard, add more plain vinegar; you may be able to keep the same peppers producing heat for several batches.
To make the mustard, put powder in a pint mason jar to fill it a little more than halfway. Pour in hot vinegar (hot refers only to the flavor; do this at room temperature) and stir it, adding more vinegar as the powder dissolves to a paste. It’s important to get all the powder incorporated into a uniform slurry and not lurking in dry pockets at the bottom of the jar. Lift the jar and peek at the bottom to make sure there are no dry areas; you can use a chopstick to get into the corners and mix everything. I think from this photo that I have one of these pockets at the bottom of my jar, and I will go stir it up right now.
Reconstituted mustard will thicken up as it sits in the fridge after mixing, so you want the slurry to be more liquid than the finished mustard. If it gets too thick, stir in a little more vinegar or white wine. I haven’t tried this, but if you’re a fan of sweet mustard you could stir in some honey or brown sugar.
Make sure to store the mustard in glass. It’s better to use a plastic cap than a regular metal Mason jar lid, because the mustard fumes will eventually rust the metal. And make sure you don't rub your eyes until you've scrubbed all the pepper/mustard off your hands.