AKA Bircher Muesli. This is very like what we used to have at school; much nicer, in my opinion, than the cereal-type mueslis one can get these days.
Equal quantities (probably about 1/2 cup each) porridge oats OR muesli base, milk, orange or multi-vitamin juice, natural yoghurt.
1 apple, peeled and grated
Other fruit, fresh (ideally) or dried. I used a peach, an apricot and some strawberries. You can use frozen fruit, too, if you like - frozen rasps are good. If you want to use bananas, add them at the last minute or they'll go slimy. In the winter, sultanas or dried cranberries are very good. It needn't be the same twice!
Mix everything together, stirring well so that the apple doesn't discolour. Leave in fridge overnight. Will keep 2-3 days. Serves 3-4 people.