Kohlrabi tastes somewhat like the core of a cabbage. It's sweet and aromatic, with just a touch of bitter, but just a touch. If they're in season, they can be almost as sweet as a sweet onion. It is very popular in the middle east. If you find the small ones, they don't need to be peeled, but if they are larger than a first grader's fist, you need to take off their skin.
For this recipe, you should try to get them with their greens. Just trim the tough spine from them up to where the leaves start.
If you can't get them with the greens, you can use either cabbage or spinach to make up for their loss.