1. Preheat your oven to 200°C/180°C fan/gas 6.
2. Peel and core the apples. Cut into thick slices and toss them in a bowl with the sugar, and cinnamon if using. Place them in a shallow baking dish approximately 23cm x 18cm.
3. To make the crumble topping, start by sifting the flour into a large bowl. Rub the butter into the flour until it resembles coarse breadcrumbs, then stir in the sugar.
4. Sprinkle the crumble mixture evenly over the apples. Make sure that all the apples are completely covered, right up to the edges of the dish. Press the crumble mixture down quite firmly and then use a fork to lightly rake the surface.
5. Place the crumble on the centre shelf of the oven and bake for 35-40 minutes, until the apples are bubbling and the topping is crisp and golden brown.
6. Leave it to rest for 5 to 10 minutes before serving with custard, cream or ice-cream.