Yellow mustard has been a popular condiment since the 1904 St. Louis World's Fair, when French's introduced what it called “cream salad mustard.” It is easy to make your own yellow mustard at home with a few spices and vinegar. You can even experiment with different concoctions to achieve new flavors. Different vinegars, for example, will lend different flavors to the finished product.
Combine all of the ingredients in a small saucepan.
Whisk the mixture until it's smooth over medium heat, bringing it to a low boil.
Reduce the heat to medium-low after the mustard comes to a boil, simmering for about five minutes and stirring frequently.
Remove the mustard from heat and place it in a covered container in the refrigerator for several hours until fully cooled and thickened.
To use mustard seeds instead of mustard powder, toast the seeds in a skillet over medium heat for about one minute or until they start to pop. Remove them from heat and cover them to stop the popping, then grind the seeds into a powder with a spice grinder or mortar and pestle.