Preheat the oven to 190°C/fan 170°C/gas mark 5.
Start by making the filling. Put the apples, golden caster sugar, cinnamon, lemon juice and plain flour in a saucepan with 50ml of water. Stir well and cook on a medium heat for roughly 15 minutes until the apples have softened nicely, stirring all the time.
For the topping, add the oats, demerara sugar, self-raising flour and chopped hazelnuts to a large bowl. Pour over the melted butter and vanilla extract and stir to combine.
Once the apples have softened and the topping is combined, lightly grease a heat-proof dish (roughly 28 x 25cm and at least 5cm deep) with a little butter. Fill the dish with the softened apples and top with the crumble mix making sure all the apples are completely covered. Slide onto the middle shelf of the oven and bake for 40 minutes or until the topping is golden brown.
To serve, top the apple crumble with vanilla ice cream or custard.