Mustard

This humble plant might have remained a common (if unremarkable) spice in Kytheria were it not for the species' predilection for colder climates. Though humans living in the northernmost and southernmost reaches of the Three Sisters make use of mustard, the dwarves with their cities in generally higher altitudes have elevated mustard to immense cultural significance. It's not enough to say that 'mustard' is, in general, the flavor of dwarven cooking- millennia of cultivation have lead to different strains and styles becoming favored by different Houses and regions, such that a connoisseur of dwarven cooking could tell you the city of a cook's origin by what mustard they prefer and how they work it into their dishes. The seeds are used with vinegar (often malt) to create a tremendous array of sauces, while mustard greens find their way into most stews prepared by dwarven families. Along with its close relatives cabbage and turnips, mustard is one of the Three Dwarven Sisters of agriculture, and is so ubiquitous that it's become synonymous with hospitality. Coffee with fried turnips and mustard is the traditional offering of dwarven hosts.
Though it tends to be grown at higher altitudes (which are generally cooler), lowland dwarves maintain their culture's affinity for mustard, and it's one of the most heavily traded items between the two subcultures. Bearing in mind the House and regional differences, lowland dwarves tend to prefer a thinner sauce used for dipping and serve mustard on the side for breaded and fried foods, while upland dwarves often use a thick spread. Greens are widely used by both, and dwarven farmers carefully rotate their crops to maintain optimal soil conditions, ensuring that mustard is constantly being planted at certain times of the year. The spicy ale made by the dwarves hasn't found much of a following among humans, who find the flavor combinations a bit off-putting, but this suits most dwarves just fine.