Diet is problematic for all the underground-dwelling races, and especially any that are described as having large communities. The best explanation is simply to invoke magic: they either use magic to create food and water out of nothing, or they have some magical means of farming. Either way, they eat whatever you choose to describe.
For a race like dwarves, though, which can be described as living close to the surface, the same issue doesn't apply as strongly, because they can farm on the surface and import what they can't produce themselves. And so you can have hill dwarves raising crops, plus animals such as sheep and goats which are pretty hardy.
Indeed, one could well see imagine schism forming in dwarven societies: perhaps the older generations lived deep underground and subsisted on manna for generations, while the younger generations have adopted surface foods with great gusto (malt beer, ripe meat off the bone!), leading to a conflict of culture between tradition and modernity.
(Oh, and if you want to go with the Dwarves = Scots thing: porridge made with oats for breakfast; hearty stews made with mutton, lamb, and root vegetables; and soup, lots of soup. (Apparently, Scots are the biggest consumers of soup, per capita, in the world.) And if you're going for a more modern flavour, batter everything and deep fry it.)