Method

Trim the leeks and cut the white part into 15cm (6 inch) pieces. Split the pieces in half lengthways. Finely chop the green tops. Cook the white part in plenty of water for about 15 minutes or until just tender. Cook the pasta as directed on the pack.
Melt 25g (1oz) of the butter in a frying pan and gently cook the onion and leek tops. When golden, add the mushroom and cook for about 3 minutes. Keep hot.
Place the remaining butter, the flour and the milk in a small saucepan and whisk over a moderate heat until thickened. Simmer for about 2 minutes.
Stir in most of the cheese and season with salt, pepper and nutmeg.
When the sauce is smooth, remove from heat and stir in the mushroom mixture. Season to taste.
Place the leeks and pasta in a shallow ovenproof dish. Pour over the sauce and sprinkle with the remaining cheese.
Place under a hot grill until golden. Serve immediately.