- Preheat the oven to 190 C / Gas 5.
- Rub the butter or margarine into the flour until it looks like breadcrumbs. Stir in the oats and sugar and make sure they're well mixed in.
- Arrange the apple slices in an oven-proof dish, then add the sultanas. Sprinkle the crumble topping over the apple slices. Place the oven-proof dish on a baking tray so if anything bubbles over, it will land on the tray, not in the oven.
- Bake until the apple is soft and the crumble is golden, 25 to 30 minutes.
Be creative and experiment with other fruits, such as pears, blackberries, apricots, raspberries, peaches, nectarines or plums. If you can't get fresh fruit, use canned fruit.