To make the crumble, put the flour and butter in a food processor and pulse to rough breadcrumbs. Stir in the sugar and a big pinch of salt.
Heat the oven to 190C/fan 170C/gas 5. Cut the apples into quarters and remove the cores. Thickly slice and toss with the lemon juice, 150g of the sugar and the cinnamon. Put the apples in an ovenproof dish and scatter over the crumble mix, spreading it evenly. Sprinkle over the rest of the sugar.
Bake for 40 minutes or until golden on top and bubbling around the edges. Serve with custard or ice cream.
