Preheat your oven to 190°C (170°C fan/gas 5).

In a mixing bowl, combine plain flour with cubed unsalted butter. Rub together until it resembles breadcrumbs. Stir in rolled oats, chopped hazelnuts, ground cinnamon, caster sugar, and sea salt. Spread evenly on a baking tray and bake for 20 minutes until golden brown. Allow to cool, then reduce oven temperature to 150°C (130°C fan/gas 3).

In a frying pan, layer remaining sugar and heat until it dissolves. Watch closely as it becomes caramel with a rusty penny color. Stir in butter, add chopped green apples, and cook for 10-15 minutes until caramelized. Spread out on a tray and bake for another 30 minutes.

Pour double cream into the caramel pan, cook over medium heat, and stir until smooth.

To serve, place caramelized apples on a plate, top with crumble, add ice cream, and drizzle with caramel sauce.