• Preheat the oven to 200˚C/180˚C fan/ 392˚F

Prepare And Cook The Leeks

• Clean the leeks by putting in a colander and putting under a cold tap, moving them around with your hand to seperate the layers and get all the grit out.
• Tip them into a medium saucepan with the cold water, add a pinch of salt, bring to a boil and boil gently for 5-7 minutes until just tender.
• Set a colander or sieve over a heatproof bowl and pour the leeks into it, so the leek stock goes into the bowl.
• Save 250ml / 1 c of the stock for the sauce for the pie.

Make The Sauce

• Melt the butter over a medium heat and then add the flour and stir to combine. Cook for 2 minutes.
• Add the milk a little at a time, stirring in between then add the stock. (If the sauce is lumpy then beat with a balloon whisk.)
• Add the wholegrain mustard, fresh thyme, salt and black pepper, and Worcester sauce, if using, and simmer for 5 minutes until thickened slightly.
• Stir in the cooked chicken and leeks. Heat gently for 5 minutes.

Assemble The Pie

• Pour the sauce into a 25 x 16 cm (10 x 6") oven proof dish and leave for 5 minutes to cool slightly.
• Top the chicken and leek filling with the sheet of puff pastry. Trim any excess pastry with a sharp knife and use it to decorate the pie.
• If you want a shiny golden crust then mix some egg and brush over the top; otherwise use a little milk and then sprinkle over sesame seeds.
• Using a small sharp knife, cut 2-3 holes in the top of the pastry to let out steam.
• Place in the oven and bake for 25-30 minutes until golden brown.