1Prepare the apple mixture: melt butter in a pan, add syrup, sugars and whiskey, stir until combined. Cook for 5 minutes until slightly reduced. Add the apples and cook until coated, remove from heat.
2Grease a baking tray and pour in the apple mixture, spreading evenly.
3Make the crumble: mix flour, oats and cold butter until it resembles breadcrumbs. Stir in sugars, almond flakes and walnuts.
4Spread crumble evenly over the apples and sprinkle with sea salt.
5Bake at 180°C for 35-40 minutes, until golden and crisp.
6Prepare the custard: heat milk with vanilla pod and seeds until almost boiling.
7Whisk egg yolks and sugar until pale, slowly pour hot milk into the mixture while whisking constantly.
8Return the mixture to the pan and cook over low heat, stirring continuously until thickened. Do not boil. Remove the vanilla pod.
9Serve warm apple crumble with custard poured over the top.