Mustard was a condiment, often used on sandwiches or other "hand-held" foods. Bernice Summerfield, for instance, once enjoyed hot dogs with sauerkraut and mustard. (PROSE: The Left-Handed Hummingbird)
Mustard could also be used as an ingredient in more elaborate recipes, such as when Benny once dined in a restaurant that served braised pig kidneys in a mustard sauce on a bed of wild rice. (PROSE: Ship of Fools)
The Eleventh Doctor had a mustard dispenser on his console. (TV: Vincent and the Doctor)