Top and tail the leeks and slice into food processor bowl. Cook in boiling salted water for 20 minutes until tender. Drain, reserving 275 ml (1/2 pint) of the liquid.

Grate the cheese and reserve.

Place the milk, reserved stock, butter, flour and salt and pepper into the processor bowl and mix together for 10 seconds. Transfer to a saucepan and bring to the boil stirring all the time.

Add the grated cheese and mustard and stir until cheese melts. Season well.

Pour the sauce over the cooked leeks.

Place the onion and blue cheese in the processor bowl and chop finely, sprinkle over the top of the dish and place under a grill to brown topping.

Serve immediately.