Preheat the oven to 180°C.
Peel and core the apples, then chop them into thin wedges. Line an oven-proof dish with the apple pieces, pour over the apple juice, then scatter the blueberries and walnuts on top.
Dust with 2 Tbsp of sugar and half of the cinnamon.
In a bowl, rub the margarine with the flour and cinnamon until you get lumpy breadcrumb-like mixture. Add the remaining sugar and oats, then mix well. Add more margarine if you think it’s too dry.
Cover the apple & blueberry layers with the crumble, slightly pressing it down with the base of a spoon.
Bake for about 20 minutes until golden.
Serve warm.