Grind the mustard seeds briefly. You don't want to powder them because you're adding mustard powder anyway. Just knock them around a bit
Add the mustard powder, salt & water and mix together. Let the mixture stand for 10 minutes to absorb the water a bit
Stir in the vinegar and sweetener
Pour into a sterilized glass jar, screw the lid on and set in the fridge to cure. This mixture will fill 2 old 150g mustard jars
Wait AT LEAST 2 days before tasting. It won't be that great at first. It needs to sit a while. It will taste horrible and be quite runny when you put it in the jar but will thicken up and the bitterness will mellow out as more time passes. I leave mine a couple of weeks to get it to how I like it
If you use hot water instead of cold, the mustard will be milder. If you leave the mustard seeds whole it will also be milder. If you leave out the mustard powder it will be milder still. You can also use beer or milk as your liquid, whatever you prefer. Using less water and more vinegar will also make your mustard a little milder. If you find that after a few days the mustard is too hot for you, spoon it into a bowl and heat it through then pour back into a clean sterilized jar when cooled. The heating will dampen the spice down. You can also add more sweetener if you want to. Putting the jar in the fridge will stop the flavour intensifying. If you like a spicier mustard, let it cure at room temperature and once your mustard has reached the desired flavour, put it in the fridge to stop the flavour mellowing more
