Place the leeks in boiling water and simmer for 10 minutes. Drain well and place in a flameproof dish.
Place the milk, plain flour and butter in a saucepan. Bring to the boil, stirring continuously. Simmer for 2-3 minutes. Add the nutmeg, mustard powder, onion granules and ground bay leaves. Add about three-quarters of the cheese. Stir until the cheese has melted.
Pour the sauce over the leeks. Sprinkle with the remaining cheese and place under a pre-heated grill until bubbling and golden brown. Or place in the oven for 20-30 minutes at 200-220 °C / 400-425 °F / Gas 6-7.