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 180C oven for 20 to 25 minutes, or until topping is crisp and browned. Serve warm with yoghurt.

For a delicious variation, replace apples and blueberries with pears and raspberries.