Cut the apples into chunks and put into a pan with the sugar and water. Cook over a low heat for 5 minutes then tip into a small ovenproof dish.
Put the flour and oats into a mixing bowl and mix well. Cut the butter into small cubes and add to the oats and flour. Mix with your fingertips until it resembles an even crumb mixture then add the sugar and mix through.
Cover the fruit with the crumble mixture. Put into the oven and bake for about 20 minutes until the crumble is golden and the apple hot.