Crab and potato pasties

Method

1. To make the pastry, put the flour, salt, lard and butter in a food processor and pulse until the mixture resembles breadcrumbs. Add the water, then pulse until the pastry comes together in a clump. (If you don’t have a food processor, rub the fats into the flour and salt with your fingers, then work in the water using a dinner knife – this will take much longer.) Tip out the pastry, knead on a work surface until it becomes elastic, then flatten into a fat disc, wrap in cling film and chill for 2 hours. 
2. To make the filling, put the potatoes into a large pan of boiling salted water and cook over a medium heat for 15 minutes. The potatoes should be tender but not falling apart – cook for a little longer if too firm. Once done, drain and set aside to steam dry. When cooled slightly, cut into 1cm cubes.
3. Meanwhile, melt the butter in a large saucepan and stir in the flour with a pinch of salt. Cook over a medium heat, stirring, until the flour smells biscuity, then stir in the mustard powder and cayenne and cook for another 1-2 minutes. Turn up the heat and gradually add the milk, stirring constantly – the sauce should remain thick while you’re adding the extra liquid (don’t add it too quickly). Once all the milk has been added, stir in the lemon zest and juice, the cubed potatoes and the crabmeat. Taste and adjust the seasoning, then leave to cool completely. When cool, stir in the chives. 
4. Roll out the pastry on a floured work surface to just thicker than a £1 coin. Use a side plate or saucer to cut 15-18cm circles from the pastry, re-rolling as necessary (you should get 5). Put a fifth of the filling onto one half of each circle, leaving a clear 1.5cm border around the edge. Brush the exposed edges with water, then fold the other half of the pastry over to enclose the filling. Press the edges together, then fold them back on themselves in 2cm sections to create the traditional pasty crimp. Brush the pastry all over with beaten egg, then transfer to a baking tray lined with baking paper and chill for 20 minutes. Heat the oven to 200°C/180°C fan/gas 6. Once firm, brush the pasties with beaten egg again, then bake for 20-25 minutes until golden and crisp. Serve hot, warm or at room temperature.