In a large pan over medium heat, melt the butter. Add the golden syrup, golden sugar, and brown sugar, stirring until combined. Stir in the whiskey and cook for 5 minutes or until the mixture has reduced slightly. Add the apples and cook for a couple of minutes until the apples are coated in the mixture. Remove from heat. Grease a large deep oven tray with butter and pour in the apple mixture, spreading it evenly.

Make the Crumble Topping: In a large bowl, mix the flour, rolled oats and cold butter and rub it into the mixture using your fingertips until it resembles coarse breadcrumbs. Stir in the golden sugar, brown sugar, almond flakes and walnuts. Spread the crumble evenly over the apple mixture, pressing down slightly to create a thick, firm topping. Sprinkle with sea salt flakes.

Bake: Preheat the oven to 180°C. Place the dish on the middle shelf and bake for 35-40 minutes, or until the topping is golden and crisp, and the apples are bubbling.

Prepare the Custard: In a saucepan, heat the milk with the vanilla pod and seeds until just below boiling. In a separate bowl, whisk the egg yolks and golden sugar until pale. Slowly pour the hot milk into the egg mixture, whisking constantly. Return the mixture to the pan and cook gently over low heat, stirring continuously until the custard thickens. Do not allow it to boil. Remove the vanilla pod.

Serve: Spoon generous portions onto plates, and serve with warm custard poured over the top.