I like to imagine it would actually be pretty tasty. Now, I'm not 100% sure of what kind of Flora thrives in the Underdark, but I assume at least some kind of Subterranean root vegetable, like potatoes would be able to thrive in the environment which could be used in a series of meals like stews, side dishes, or to be roasted, the starch could also be fermented to make alcohol like vodka (which would pretty fun). Mushrooms, at least normally can be quite edible and make for great flavor bases in broths for soup would be very common staple of the Commoner.
There are also all sorts of bugs and arachnids in the Underdark that can be used as a potential meat substitute if regular Rothé aren't readily available. Things like crickets, spiders (though, I don't think these would really be on the menu due to Lolth), dung beetles, and the sort are very edible if chosen carefully as they're very dense in nutrients... These are, in my opinion, the least appealing option for food, but many real cultures use these in their food, so it might be simply an acquired taste.
The Rothé would obviously be used as a source of meat and potentially be used to till soil with the slaves, but the female ones would be used for milk (I think I remember a post from a while mentioning that it sounded revolting but I strongly disagree) that milk could then be used to make thinks like, butter, cheeses, creams, yogurts, and custards (though I imagine only the Nobles would have access to the good stuff) I imagine it would likely have a similar taste to that of Yak milk, which has a lightly sweet, nutty floral taste. Very nice. The meats could be turned into a jerky with salt which could be fairly common and is likely richer than regular salt that Surfacers would have, making it a good resource to have.
Things like rats, bats, and other creatures could also be used, I know they sound pretty bad, but basic survival would require you steel your stomach for a few things. Rats and bats are fairly gamey, but rats are known to taste fairly similar to chicken, while bats have a taste between that of chicken and pork. Fish, frogs, and potentially eel (ell is more depending on the waters, tbh) would also be available, which is great because the entire fish is practically edible, and frogs are a good source of protein. Snails, and slugs are also available honestly, which could be used with the Rothé milk to make a soup.
I think this got kind of long, but I just really like thinking of what kind of meals the Drows could be eating
