Place apples, water and sugar in a large saucepan. Bring to the boil. Reduce heat. Cover and cook for 8-10 minutes, stirring occasionally, until apples are just tender. Remove. Stir in blueberries.
Transfer apple and blueberry mixture to a shallow ovenproof dish (6-cup capacity).
To make topping, process flour, muesli, sugar, cinnamon and margarine in a food processor until well combined. Sprinkle over apple and blueberries.
Bake in 180 °C oven for 20 to 25 minutes, or until topping is crisp and browned. Serve warm with yoghurt.