Sift the flour and salt into a large mixing bowl and add the butter. Rub the butter into the flour until the mixture resembles fine breadcrumbs. Add the measured water and mix to form a rough dough. Turn on to a lightly floured work surface and knead until smooth. Place in a food bag and refrigerate for 30 minutes.
Meanwhile, heat the oil in a frying pan and cook the onion and lamb over a moderate heat for 5 minutes, stirring occasionally, until beginning to brown. Add the potato, reduce the heat and cook for a further 2 minutes, stirring occasionally until beginning to brown.
Mix the lamb stock with the mustard and pour into the pan. Cover with a tight-fitting lid and simmer gently for 15 minutes, stirring occasionally until the potatoes are soft yet still retaining their shape, and the meat is tender. Stir in the mint and set aside to cool.
Roll out the pastry to 5 mm (¼ inch) thick, and using a 15 cm (6 inch) saucer as a template, cut out 6 rounds. Lightly brush the edges of each of the circles with a little water and place 2 tablespoons of the mixture in the centre of each. Fold up to enclose the filling and pinch and gently twist the edges to seal. Place on a baking sheet and lightly glaze each one with the beaten egg. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until the pastry is golden and crisp. Wrap loosely in foil to keep warm.