Cold chicken, pickles, seed cakes, red wine, beer, bacon, eggs, coffee, mince pie, apple tart, cheese...
Pretty much anything humans eat, and with equal relish. Mountain dwarves tend to live in Alpine regions, so you'd expect their cuisine to be similar to that of real world alpine regions - for example, Peru, Switzerland, and Tibet. Yak, alpacas, goat, mutton, potatoes, domesticated marmots or guinea pigs, barley, yogurt, cheese, quinoa, beets, legumes, and various berries are likely food stuffs. Some of these may be regional if trade is not global in your world, but in typical cosmopolitan D&D worlds most of the plants and animals would have been traded around centuries ago resulting in the sort of varied robust cuisine typically seen in the real modern world.
Dwarves are explorers, travelers, and traders and so many are broadly familiar with cuisines of other species and regions and may develop keen appreciation for other than their own native foods.
Ideally, dwarves prefer to leave agriculture to their neighbors and to lower caste dwarfs. They consider agricultural pursuits to be less intellectually fulfilling and less honorable than 'creative' trade skills like masonry, smithing, and so forth. Only relatively low caste dwarfs normally engage in farming and animal husbandry, although brewing and distilling are skills greatly prized by dwarves and considered quite honorable. As such, most successful dwarf farmers also have a 'house brand' of some sort, that they use as social cover for their agricultural endeavors ("I found I couldn't trust the hops and barley to anyone else. My suppliers just didn't have an eye for quality, and you know what they say, if you want it done right, do it yourself."). It is a very common situation to employ human tenants to work the surface land and provide food to the dwarf kingdoms.
Normally, only a small percentage of the food stuffs of a dwarf kingdom comes from deep farms in the underdark where ambient magic is sufficiently high to substitute for the sun. These fungal food stuffs and the animals that subsist on them are not considered to be as wholesome or as tasty as those coming from the surface, and are relied on mainly to extend food supplies when besieged. Generally speaking, dwarves respond to extended sieges not by merely bunkering down, but by extending tunnels out secretly in all directions so as to force the besiegers to try to cover larger and larger arrays to keep the besieged bottled up. Tunnels after all can be easily collapsed if need be, and it's difficult (barring treachery) to force passage of a long and narrow tunnel. However, while they will also try to increase their internal food supplies as a precaution, deep foodstuffs as associated closely with goblins and the hated Duergar ethnic group, and a dwarf that develops too keen of an appreciation for such food stuffs draws suspicion and even scorn. Ironically, the Duergar themselves, eschewing all comforts and aesthetics, feel much the same way - enjoying one's food too keenly is a sign of having fallen into the sins of the decadent soft surface dwellers.