Mustard was a food item offered as a side dish alongside any sliders at Dex's Diner. This was before the diner's destruction, which occurred in 19 BBY. Two types of mustard available were Bimm mustard and Boontaspiced mustard.
Gregory Walker mentioned mustard in "Dining at Dex's", a release exclusive to Hyperspace. This article details the in-universe menu of Dex's Diner. In the real world, mustard is a well-known condiment created using the ground seeds of mustard plants.
- Dining at Dex's on Hyperspace (article) (content obsolete and backup link not available) (First mentioned)