Winter crumble tart

Name: Winter crumble tart
Ingredients: 225g/8oz plain flour, plus extra for dusting; 125g/4½oz chilled butter, cut into cubes; 30g/1oz caster sugar; 1 large free-range egg, beaten; 4 large Bramley apples (about 1.2kg/2lb 10oz), peeled, cored and cut into small chunks; 150g/5½oz caster sugar; 275g/10oz fresh blackberries; 175g/6oz plain flour; 100g/3½oz butter, at room temperature, cut into cubes; 50g/1¾oz rolled oats; 100g/3½oz demerara sugar; 50g/1¾oz hazelnuts, roasted and chopped; custard or double cream, to serve