Pre-heat the oven to 200C / 400 F / gas mark 6.
Combine the flour, oatmeal, and butter in a food processor or large bowl, and pulse briefly, or rub with your fingertips, until the mixture resembles very coarse breadcrumbs with a few larger lumps.
Add the sugar and stir through.
Put this in the freezer for 10 minutes, or if making ahead, in the fridge until you're ready to bake.
Meanwhile, toss the apple slices with the cornflour and caster sugar until all coated.
Add a splash of water (25 - 50ml) to the bottom of your baking dish then layer in the apple slices.
Spoon the crumble over the apple slices and sprinkle with the rolled oats.
Bake for 30 minutes until the apples are just soft and the crumble top is golden and crunchy.