Peel, core and slice the apples. Put apples, lemon juice and butter into a pan, cover, and simmer slowly until pulpy. Add sugar and semolina, and bring slowly to the boil. Cook gently for five minutes or until the mixture has thickened. Remove from the heat and leave until completely cold. Divide the pastry into two pieces. Roll out one portion and with it line a 7-8 inch heatproof plate. Spread with half the apple-filling to within half an inch of edges. Sprinkle with currants and add treacle, and then top with the remaining filling. Roll out the rest of pastry into an 8-9 inch round, moisten edges with water and cover the pie. Press the edges well together to seal, and knock up with the back of a knife. Brush the top with beaten egg or milk and then bake towards the top of the oven set at 425°F, 220°C, gas mark 7 for 25-30 minutes or until pale gold.