Mustard was a Human condiment made primarily from the ground seeds of the mustard plant.
Mustard was served, alongside ketchup and salt and pepper, at the Pine Tree Bar and Grill in Carbon Creek in 1957. (ENT: "Carbon Creek")
Sisko's Creole Kitchen offered shrimp remoulade: gulf prawns with a dipping sauce of mustard, capers, shallots, and onions, as a special in 2372. (DS9: "Paradise Lost")
When Jadzia Dax and Worf briefly canceled their wedding in 2374, Miles O'Brien ordered a double Altair sandwich, with no mustard. (DS9: "You Are Cordially Invited")
Although never mentioned, the Klingons apparently make mustard, at least according to Penny Juday and the Star Trek: Deep Space Nine art department. Juday shows the container in the DS9 Season 1 DVD special feature "Secrets of Quark's Bar".
A bottle of Klingon mustard