Preheat the oven to 200°C (400°F), or gas mark 6.
Peel, core, and cut the apples into quarters, then chop them into roughly 3cm pieces.
Place them in a saucepan over a medium heat, adding 100g of sugar and a few light shavings of lemon zest.
Cover with a lid and cook for about five minutes, or until the apples have softened. Remove from the heat and allow to cool slightly.
While the apples are cooling, dice the butter and place it in a large mixing bowl with the flour.
Using your fingertips, rub the butter and flour together until the mixture resembles coarse breadcrumbs.
Once this is achieved, stir in the remaining sugar to create a little texture in the crumble topping.
Transfer the softened apples to a 25cm by 30cm baking dish, then evenly distribute the crumble mixture over the top.
Place the dish in the oven and bake for 25 to 30 minutes, or until the crumble topping is golden brown and the filling is bubbling.
Serve with a generous portion of custard or some vanilla ice cream for an extra indulgent treat.