1. In a saucepan melt the butter on a medium heat and add the flour stirring constantly until you have a roux. Cook gently for 3 mins then slowly add the stock 1 ladle at a time stirring continuously until you have a lump free sauce.
2. On a low heat cook the sauce for 10-15 mins to cook out the flour taste. Add the chopped thyme and double cream and season to taste. Allow to cool.
3. In an oven proof rectangular dish add the turkey and leeks, pour over the sauce then roll your pastry over making sure it overlaps the edges.
4. Rest in fridge for 10 mins. Brush with egg yolk and cook in a pre heated oven at 190°C for 35-40 mins.