Put the frozen berries into a large processor or blender with the sugar, apple juice and raspberry yogurt. Whizz until really smooth.
Rest a sieve on top of a large jug and pour the smoothie through. Stir the mixture with a metal spoon to help it through – this takes a while but removes all those raspberry seeds. Chill until needed or put into a Thermos flask to take on a picnic. Don’t forget the plastic cups!