In a bowl, rub the butter into the flour and sugar until it resembles breadcrumbs. Add the egg and water, a tablespoon at a time while you knead it into a smooth dough (you can also do this in a mixer).
Cover and refrigerate 15-30 minutes.
Meanwhile, peel, core and chop the apples. Add them to a large pot with water and sugar and bring to a boil. Cook, stirring occasionally, until the apples break down. Set aside to cool.
Divide the dough in two and roll out on a well-floured surface two slabs that will cover a 20x30cm slice tin.
Place one layer of the dough in the tin, cover with the apples, and layer the second dough piece over.
Sprinkle with sugar and bake 40-60 minutes - just until the top is golden brown. Leave to cool thoroughly before slicing.