Preheat the oven to 180°C.
Peel and core the apples, then chop roughly into bite-sized chunks.
Put the fruits, water and half of the sugar in a saucepan. Bring to the boil and cook for 5 minutes until tender.
Transfer to an oven-proof dish.
Rub the margarine/butter with the flour and cinnamon until you get fine breadcrumb-like crumbles, then add the oats and the leftover sugar.
Cover evenly the fruit mixture with the crumble, slightly pressing the top with a spoon.
Bake for 20-25 minutes until golden and crispy.
Serve it warm with vanilla ice-cream, custard or Greek yoghurt.