Combine dry and wet ingredients in a pan. Bring to a boil, turn down heat and simmer for 2 minutes. While the mixture is still warm, pack it into clean jars, pushing out air spaces, and refrigerate. Keep mustard fresh by placing a slice of fresh lemon inside the jar, on top of the mustard. Replace the lemon with a fresh slice every few days.
Water makes mustard hotter.
If you prefer a mellow version instead, use oil or mayonnaise instead of water. Horseradish can also be put in potato salad and cocktail sauces. The trick is to grate the fresh root directly into lemon or vinegar so that it gets as little exposure to air as possible. If all this sounds like too much work, you can buy the root already dried as wasabi powder. This is the amazingly hot paste served with some Japanese dishes.