Peel, core and slice the apples. Cut the rind of the lemon and orange into pieces. Put the apple and rind into a saucepan with a little water, and cook over a low heat until soft. Remove the rind, and add the sugar to sweeten. (Or use tinned solid pack apples A10 size)
For the crumble topping, sift the flour into a bowl.
Cut the fat into cubes and rub into the flour until the texture resembles breadcrumbs, then mix in the sugar.
Spread the crumble evenly over the fruit.
Bake in the preheated oven for 35–40 minutes until lightly browned.