For the pastry, freeze the butter for 10 minutes. Place the flour in a large bowl with a pinch salt. Using a coarse grater, grate the butter into the flour, tossing every so often to coat the butter in flour to avoid clumping. Using a round bladed knife, gradually add 4-5 tbsp cold water until it comes together into a soft dough. Knead lightly, wrap in clingfilm and chill for 30 minutes. Roll out to a 30x15cm rectangle and fold down the top short edge of pastry 2/3 of the way down and then the bottom edge up over the top. Chill for 15 minutes and repeat.

Meanwhile, prepare the filling. Boil the carrots for 3 minutes, add the peas for a further 1 minute and drain well.

Melt the butter in a medium saucepan and add the flour, cook for 1 minute before whisking in the milk and mustard. Bring to the boil, stirring until thickened, season. Stir in the vegetables, turkey, gammon and 75g Walnuts. Allow to cool.

Preheat the oven to 200oC, gas mark 6.

Roll 2/3 of the pastry into a 30cm circle and line the base of a 20cm loose bottomed tin, allowing the pastry to come 4cm up the sides. Add the filling.

Roll out the remaining pastry to a 22cm circle and brush the edge with a little beaten egg. Place on top of the filling and crimp the edges to seal. Brush the top with egg and sprinkle with remaining chopped Walnuts. Place on a baking tray and bake for 50 minutes, covering with foil towards the end if looking dark. Allow to cool before removing from the tin.