Method:

1. Preheat the oven at 190°C / 375°F / gas 5.
2. Heat the fat in a large, heavy-based frying pan on a low heat and add the leek. This should cook down slowly for 5 minutes so that the leeks soften but don't brown.
3. Add the mushrooms and sprinkle over the fresh thyme leaves (leave the springs out) and fry down for a further 3-4 minutes.
4. In a separate pan add the chicken broth and bring to a light simmer.
5. Add the meat to the mixture and scatter over the flour ensuring nothing browns but the mixture merges into a chunky batter.
6. Stir in the mustard until it's well blended.
7. Slowly add the chicken broth, season generously and stir continuously until the sauce is a thick luscious consistency.
8. Stir in the crème fraîche to lighten the mix if you wish, but it's not required.
9. Dust a large clean surface lightly with flour and roll out the pastry to around 1cm thick. Ensure the rolled piece of pastry is larger than the top of the pie tin you intend to use.
10. Fill the pie tin with the mixture ensuring everything is evenly distributed. You want the tin 80-90% full, not overflowing.
11. Cover the tin with the pastry and press down the edges so it's sealed.
12. Cut any excess pastry away from the outside of the tin edges and roll out and cut into any festive shapes you wish. You could even utilise a cookie cutter if you have one lying around!
13. Make sure there's a small hole cut in the centre of the pastry to let any steam out. I just use a sharp knife and briefly stab the top.
14. Once the crust is decorated, mix the beaten egg and milk vigorously and use a brush to coat the pastry so it goes all golden and crispy in the oven.
15. Place in the oven and bake for 35-40 minutes until the pastry is a deep golden colour.
16. Serve with leftover pan-fried greens in butter: Sprouts, cabbage and peas would work wonderfully.