Preheat the oven to 200°C/Fan 180°C/Gas 6. Roll the pastry into 2 sheets about 20 × 30cm, and select 2 large pastry cutters – one about 2cm larger than the other. Cut out 4 pastry discs with the larger cutter, and 4 with the smaller one.
Add the butter to a pan and sauté the mushrooms and leek for 4-5 minutes. Add the white wine, tarragon, and zest, and season with salt and white pepper.
Cook for 3-4 minutes until the wine has reduced, then add the cornflour. Stir, then remove from the heat and allow to cool.
Put a small handful of the filling in the centre of each smaller pastry disc. Then wet the edge, lay the larger disc on top and gently press with a fork to seal. Score and make a small hole in the top for steam to escape.
Lay on a lined baking tray. Brush with egg, bake for 20-25 minutes, and enjoy.