Preheat the oven to 180C.
First make the crumble (double up and save half the mixture in the fridge, it'll keep for about a week). The easiest way to do it is in a food processor. Simply roughly chop the butter and whizz with flour, then add sugar and a wee pinch of salt, and whizz again. Or make by hand: rub flour with cubed cold butter until crumb-like. Mix with sugar and pinch of salt.
Simply slice the peeled apples into a medium baking dish. After one layer is in, sprinkle with sugar. Do another layer of apple and then sprinkle again.
Tip the custard on and smooth over.
Top with crumble mix. Bake for 30-40 minutes. The longer you bake, the softer the apple will be.