Preheat the oven to 200°C, gas mark 6. Tip the flour and poppy seeds into a large bowl. Add the butter and lard and rub with your fingers until it resembles large breadcrumbs. Add the Cheddar and about 8-9 tbsp of cold water until it comes together in a soft dough, then knead briefly. Wrap in clingfilm and place in the fridge to rest for at least 30 minutes.
To make the filling, heat the olive oil in a large sauté pan, add the leeks and gently cook for 10 minutes with the lid on, until softened but not coloured. Add the sage and cook for a further 2 minutes, then stir in the flour, and season generously.
Slowly pour in the wine and then the stock, until a thick sauce is formed. Add the cooked turkey and ham and stir in the crème fraîche.
Tip the filling into a 2-litre lipped pie dish and leave to cool completely.
Meanwhile, roll out the pastry to 5mm thickness and cut to just bigger than the size of the dish. Brush the edges of the dish with water and use the pastry trimmings to cover the lip. Place the rest of the pastry on top of the pie, pressing onto the pastry lip. Slash the top to allow steam to escape and use any leftover pastry to decorate. Brush with egg yolk and place in the oven to bake for 35-40 minutes, until golden.