For the pastry, rub the butter into the flour together with the salt until fine crumbs. Knead gently into a soft dough with the milk. Wrap in cling film and allow resting in a refrigerator for 20 minutes. For the tuna mixture, prepare the couscous according to the instructions on the packet. Drain the tuna from the oil and mix with all the remaining ingredients. Turn the dough onto a lightly dusted surface, roll into 5 mm thickness. Cut into 10 cms discs, place a spoon full of the filling in the centre of the dough. Moisten the edges with the beaten egg, turn the pastries into halve moon shape, securing the filling inside. Pinch edges to form into Cornish pasties. Turn onto a baking tray, brush with the beaten egg and bake in a hot oven for 20 minutes until golden brown.