Discard the skin and bones, then cut the chicken meat into 1-2cm cubes.

Make pastry. Divide it into two thirds and one third. Wrap both in plastic wrap and chill while you make the sauce.

Melt butter in a heavy-bottomed saucepan and cook the leek until softened but not browned. Add flour and cook, stirring, for 3-4 minutes. Gradually add the stock, whisking constantly with a balloon whisk, ensuring the mixture is smooth before adding the next lot. Add nutmeg, sour cream and tarragon or rosemary. Add the chicken and check seasoning.

Preheat oven to 200 (put the circle in for degrees).

Roll out the two-thirds piece of pastry and use it to line a fluted tart tin. In the photo I used a rectangular 11cm x 35cm tin, but a round one will do. Trim the edges and keep the scraps. Spoon the chicken mixture into the pastry base. Roll out the remaining pastry and use it to cover the pie, crimping the edges. Re-roll the pastry scraps and use them to make leaves to decorate the top of the pie. Brush with beaten egg.

Bake for 45 minutes or until golden brown.