Place the kohlrabi and leeks in a pan on the stove and fill with water until of the vegetables are covered. Put the thyme, butter and salt in the pan and cover completely with a lid, allowing some steam to escape. Boil the liquid gently for 20–30 minutes.
When the vegetables are tender, remove from the pan and place in an 8- by 8-inch baking pan. Pour the liquid over the top and cover with foil. Place in a 325° oven for 20 minutes. Remove from the oven, take off the foil and sprinkle with the cheese. Bake for another 20 minutes. Serve hot.