Preheat the oven to 180°C.
Peel and core the apples, then chop them into bite-sized pieces. Arrange them into a lightly greased baking dish, then scatter the blueberries and walnuts around them.
Squeeze the lemon juice on top, then dust with about half of the sugar and cinnamon.
In a bowl, rub the margarine/spread with the flour, oats, ground walnuts, ginger and the remaining sugar & cinnamon until you get lumpy, sticky crumble. Add more butter/margarine if it feels too dry.
Cover the fruits with the crumble, slightly pressing it down with the base of a spoon.
Bake for about 20-25 minutes until golden.
Serve warm.