Mix the white flour, oatmeal, sugar, salt and cinnamon and cardamom.
Crumble to cold diced butter into the flour until the mixture resembles coarse breadcrumbs.
Reserve cold until needed.
Filling
Peel and dice the apple. Toss them with the sugar in a greased ovenproof dish.
Spread the cold crumble mixture on top.
Bake the crumble for 30 min at 180 °C in a preheated oven or until the top is cooked and golden.
Custard
Mix some of the cold milk with the white sugar, vanilla flavoured sugar and custard powder.
Bring the remainder of the milk to a simmer.
Pour in the cold milk and sugar mixture and bring to the a gentle boil while stirring until the custard just boils and thickens noticeably. Taste and serve warm with the crumble.
Stuff to know
This quantity will fill three 2-inch gastro baking trays measuring 30x50 cm and makes about 100-120 portions. The amount of sugar added to the diced apple is to taste and may depend on the sweetness of the apples used. If desired, add chopped nuts, raisins, blackberries or a pinch of green cardamom to the filling.
Recipe by Jeroen Valkenier Recipes at https://jpvalkenier.com/apple-crumble-with-custard-xl/