Bring a 3 or 4-quart pot of water to a boil, and give it a good couple pinches of salt. Add the kohlrabi and boil until tender but not falling apart, about 8 minutes. Drain and set aside on the plate you plan to serve the kohlrabi on.
Melt the butter in the same pot, and then sprinkle the flour over. Whisk constantly until the flour is cooked and turns a light tan color, about 3 minutes. Gradually add the half-and-half, whisking the entire time. Turn the heat down to a simmer, and keep whisking until the sauce thickens, about 10 minutes. Pour the hot sauce over the kohlrabi, and grate a dusting of nutmeg with a thin grater over the entire dish. Season with salt and serve!