Trim most of the green part off the leeks (use in the stock pot). Leave the white parts whole, slit the top and wash well under cold running water. Cook in a little boiling salted water in a covered saucepan until just tender, 15 minutes approx.
Meanwhile put the cold milk into a saucepan with a few slices of carrot and onion, 3 or 4 peppercorns and a sprig of thyme or parsley. Bring to the boil, simmer for 5 minutes, remove from heat and leave to infuse for 10 minutes. Strain out the vegetables, bring the milk back to the boil and thicken with roux to a light coating consistency. Add the mustard and two-thirds of the grated cheese, but keep the remainder of the cheese for sprinkling over the top. Season with salt and freshly ground pepper, taste and correct the seasoning if necessary.
Drain the leeks well, arrange in a serving dish, coat with the sauce and sprinkle with grated cheese mixed with a few Buttered Crumbs (if you have them to hand). Reheat in a moderate oven, 180°F/regulo 4, until golden and bubbly.