Making mustard is very simple: grind the seeds and add liquid.
Both Chinese and English mustard is nothing more than water and yellow mustard powder. But there are some things you need to know to make great mustard. Most importantly you need cold liquid. What gives mustard its bite is a chemical inside the seeds reacting with cool or cold liquid. You also need to break the seeds to get at the fiery chemical. Heat damages this reaction, however, so to make a hot mustard use cold water, and warm water for a more mellow mustard. If you sauté mustard seeds the have a nutty slightly bitter flavour, but not the mustard heat.
This reaction is volatile, too. Left alone, your mustard will lose its bite in a few days, or in some cases even hours. But adding an acid, most often vinegar, sets the reaction in place — this is what happens with horseradish as well. Adding salt not only improves the flavour, but also helps preserve the mustard, too. Always add water or a non-acidic liquid first, let the mixture sit for 10-15 minutes or so, then add the acid (vinegar, verjus, lemon juice, etc). Add salt to taste, but it’s typically about 1 to 2 teaspoons per cup of prepared mustard.
VERY IMPORTANT NOTE: You cannot eat it right away. Mustard needs to marinate to loose the bitterness. If you try it right away you will taste the bitterness, then try it a day or two later.
Grind the whole mustard seeds for a few seconds in a spice or coffee grinder, or by hand with a mortar and pestle. You want them mostly whole because you are using mustard powder, as well.
Pour the semi-ground seeds into a bowl and add the salt and mustard powder. If using, add one of the optional ingredients, too.
Pour in the water or beer, then stir well. When everything is incorporated, let this sit for up to 10 - 15 minutes. The longer you let it sit, the mellower the mustard will be. When you’re ready, pour in the vinegar.
Pour into a glass jar and store in the fridge. It will be runny at first, but it will thicken up overnight. Wait at least 12 hours before using. Mustard made this way will last a year in the fridge.