Preheat the oven to 200°C/Fan 180°C/Gas 6. Put all the apples in an ovenproof dish, add the sugar and cinnamon and mix well.
To make the topping, mix the flour and caster sugar in a bowl, then add the butter and rub it into the dry ingredients with your fingertips. The mixture should have the texture of coarse breadcrumbs. Add the flaked almonds and mix well.
Sprinkle the topping over the apples and bake the crumble in the preheated oven for about 30 minutes or until the top is golden brown and the apples are cooked.
Serve with cream or custard.
You can add 50g of oats to the crumble mixture for a bit of extra chewiness.