Sift the flour and a pinch of salt together into a mixing bowl. Rub in the butter until the mixture resembles fine breadcrumbs. Sprinkle 6 tablespoons chilled water evenly over the surface and start bringing the dough together.
Add a little more water if the mixture is too dry. Gather the dough together, then lightly knead on a floured surface for a few seconds until smooth. Wrap and chill in the fridge for 20 minutes.
Make the filling:
Put 25g butter in small pan and add onion, leek, salt and pepper and fry gently for 1 to 2 minutes.
Add the lamb, fry gently for a further 2 minutes.
Add the diced potato and stock, simmer for 5 minutes. Add the mint sauce. Set aside to cool.
Divide the pastry into eight pieces, then roll out each piece on a lightly floured surface to a 22cm round. Spoon an equal amount of the filling onto the centre of each pastry round.
Cover half of each pasty circle with the filling. Moisten pastry edges and fold pastry over the filling. Use your fingers to crimp the pasty all along to try to eliminate any leakage when pasty is cooking. Brush with milk or beaten egg before placing in the oven.
Cook in a pre-heated oven at 230 C / Gas 7 for 10 minutes and then turn down to 190 C / Gas 5 for another 20 to 25 minutes. Pasty should be hard and golden brown.