LEEKS - Discard the green part of the leeks and chop the white part into thin slices. Rinse in a sieve under the tap. Melt the butter in a large shallow saucepan. When it foams add the chopped leeks and stir until they are well coated with the butter. Turn the heat down and cook gently for about 10 minutes stirring occasionally to prevent the leeks sticking to the pan. SAUCE - Sprinkle the flour over the leeks and continue cooking for 1 minute. Add the milk slowly, stirring continuously until you have a smooth sauce. Stir in the crème fraîche, mustard and tarragon leaves (or fennel seeds) and cook for a couple of minutes. Add the chicken, mix well and turn the heat off. Leave to cool. 190˚C - TURN THE OVEN ON TO 190 ˚C/170˚C FAN/GAS 5 PASTRY - Crack an egg into a bowl, add a small pinch of salt and beat lightly with a fork. Shake some flour on to a clean kitchen surface or board. Unfold the sheet of puff pastry on to it. ASSEMBLE - Pour the creamy chicken into a pie dish and brush the rim of the dish with the beaten egg. Place the pastry over the dish and brush it with the egg. BAKE - Place the dish in the oven and bake for 25 – 30 minutes or until the pastry has turned golden. SERVE