Place the butter in a large non-stick pan.
When melted, add the sliced leek and fry for 7 minutes until soft.
Tip the leeks into a bowl and set it aside.
Add the chicken to the same pan and fry on a medium heat for 5 minutes, turning regularly until no longer pink.
Return the leek to the pan and stir in the flour on the heat, for a minute or two until it disappears into the buttery veg.
Slowly add the stock a little at a time, bringing it to the boil and stirring continuously before adding more.
When all the stock is used up and the sauce is smooth, stir in the milk, thyme and mustard.
Pour the filling into an oven proof dish approximately 30cm x 20cm
Squash each roast potato, push your thumbs into the centre to break it up a little.
Push the squashed roasties in between the chicken and leek, so they're equally spaced. If you have smaller gaps to fill, you can tear a couple of them in half.
Heat the oven to 190C (fan), 375℉
Roll out the pastry, before cutting it into equal sized squares approximately 4cm x 4cm or 8in x 12in
Place the pastry squares on top of the filling, slightly overlapping each other so the top is fully covered.
Brush the pastry with beaten egg, before placing it in the oven.
Bake for 30 minutes or until the top is crisp and golden and the filling is cooked through.