Lightly dust pastry and work surface with flour. Lay pastry on a flat surface. Place pie tin, upside-down, on top of the pastry and use the edge of the tin as a guide to carefully cut a piece of pastry to fit on top of the pie. Place pastry in fridge while you prepare the filling.
Heat 1 tablespoon of olive oil in a large frypan over high heat. Add chicken pieces and cook for 5min minutes until browned. Remove from pan, reduce heat to low.
Add butter and remaining oil then add leek, cover with a lid and gently cook for 10 to 15 minutes or until leek is soft. Add 1 tablespoon flour, stir to combine, cook for 1 minute then add the stock and verjuice and stir to combine. Allow to simmer for 5 to 10 minutes or until mixture has thickened.
Remove from heat, stir in chicken pieces, cauliflower florets, cream and truffle oil. Season to taste with salt and pepper then spoon mixture into pie dish, place in the fridge to allow the filling to cool before adding the pastry lid.
Top filling with the pre-cut piece of pastry, use a fork to press the edges down. Brush the top with the beaten egg, then pop into the oven and bake for 30 minutes or until pastry is golden brown. Allow to sit for 5 minutes at room temperature before serving.