Remove the leaves from the kohlrabi and set aside. Trim the stems off the bulbs and peel them. Cut bulb into ¼-inch-thich slices. Place the water or stock in a pot and bring to a boil. Add the kohlrabi and simmer until tender-crisp, about 10 to 15 minutes. Remove the kohlrabi slices with a slotted spoon and set aside. Add the leaves to the pot and simmer until soft, about 5-7 minutes. Reserve a cup of the cooking liquid, then drain the leaves and chop them.

Heat the oil in a saucepan. Add the flour and whisk briskly for about a minute, until combined. Add the hot cooking liquid and continue whisking until a thick, smooth, lump-free sauce forms. Fold the kohlrabi slices and leaves into the sauce. Add salt, pepper, nutmeg, and chopped herbs. Taste and adjust seasonings. Serve hot.