Smothering is a way of cooking vegetables with a little fat and the least possible amount of water, in a covered pan (I guess that's the smothering bit) until very, very tender. Kohlrabi holds together well, adding its own natural sweetness.
Place the leeks, kohlrabi, carrots and garlic into a wide shallow pan which will take them in a single layer. Tuck the herbs down among them.
Pour in enough water to come about 1.5cm/½in up the sides of the pan. Season with salt and freshly ground black pepper and dot with butter.
Bring up to the boil, then reduce the heat to the absolute minimum. Cover the pan with a lid or foil and leave to cook very gently for about an hour, stirring occasionally to make sure that it doesn't catch. If necessary add an extra splash of water, or if it ends up too watery, uncover and boil the water off. Either way, you are aiming to end up with meltingly tender vegetables, perhaps slightly patched with brown towards the end of cooking, with little more than a few tablespoonfuls of syrupy liquid left in the pan. Serve warm.
