Preheat the oven to Gas Mark 5, 190ºC, fan 170°C. Peel, core and thinly slice the apples, and place in a 1litre pudding basin. Sprinkle with a little of the sugar. Pour over 2tbsp boiling water.
Place the flour, sugar and oats in a mixing bowl. Add the butter and rub into the mixture until it resembles breadcrumbs. Sprinkle topping over the apples and bake for 40-45 minutes until golden brown. Serve with a spoonful of homemade custard.