Preheat the oven to 200°C/400°F/gas 6. Peel and core the apples, then quarter and chop into 3cm chunks. Place in a saucepan on a medium heat with 100g of sugar and a few fine gratings of lemon zest. Pop the lid on and cook for 5 minutes, or until the apples have softened. Remove the heat and leave to cool a little. Meanwhile, cube the butter and place in a mixing bowl with the flour. Rub together with your fingertips until it resembles breadcrumbs, then scrunch in the remaining sugar to add a little texture. Transfer the apples to a 25cm x 30cm baking dish and sprinkle over the crumble topping. Bake in the oven for 25 to 30 minutes, or until golden and bubbling. Delicious served with vanilla custard.