Preheat your oven to 190C/375F/Gas 5. Put your bacon in a large pan on a high heat and add your thyme leaves. Add a lug of olive oil and let it all fry off for a few minutes. Add all of your prepped leeks and fry them off for about 3 minutes. Add a pinch of salt and pepper, then pop the lid on top, turn the heat down to medium heat and let them cook away gently for 30 minutes, stirring every 5 - 10 minutes to make sure they don't catch. There's going to be enough moisture in the leeks to keep them happy in the pan so they should be soft and melt in your mouth once they're done.
When your leeks are ready, add the turkey meat to them and stir. If you've got a bit of stuffing mixed in there you can put that in too. Add the flour, mix in well then pour in your stock and stir again. Add the crème fraiche then turn the heat up and bring everything back up to the boil. Have a taste and add a bit more salt and pepper if it needs it then turn the heat off. Pour the mixture through a sieve over another large empty pan and let the wonderful gravy from the mixture drip into the pan while you roll out your pastry.
Get a deep baking dish roughly 22 x 30cm. Dust a clean surface and rolling pin with a bit of flour and roll your pastry out so its about double the size of your dish. Crumble the chestnuts over one half of the pastry, then tear a few of the sage leaves over the chestnuts. Fold the other half of the pastry on top and then roll it out carefully and evenly so you have a rectangle big enough to cover your baking tray. Don't worry if a few bits stick out here and there. Spoon that thick leek mixture from your sieve into the pie dish and spread it out evenly. Lay your pastry on top, tuck the ends under then gently score the pastry with your knife. Add a pinch of salt to your beaten egg then paint this egg wash over the top of your pastry. Pop your pie in the oven for about 35 minutes or until the pastry is puffed up and golden brown. When the pie is ready, re-heat the lovely gravy and serve with your pie, along with some peas tossed butter, lemon, salt and pepper and everyone is happy!