Mustard is most often used as a condiment on meat, especially cold meats. It is also used as an ingredient in mayonnaise and vinaigrette, in marinades and barbecue sauce. It can also be used as a base for salad dressing when combined with vinegar and/or olive oil. Mustard is a popular accompaniment to hot dogs and Bratwurst.
Dry mustard, typically sold in tins, is used in cooking and can be mixed with water to become prepared mustard.
Dry mustard seeds can be fried in oil to impart their nutty flavour in dishes such as bombay potatoes.
The edible leaves can be eaten as mustard greens.
The following was taken from the wonderful book, Food in England:
Mustard secret kept
English mustard was fist made at Durham about 1730, and the recipe was kept secret for many years by an old woman named Clements, of Durham.
She ground the seeds like wheat and made a little fortune by going from town to town on a packhorse. She even secured the patronage of George I.