Preheat the oven to 180°C.
Peel and core the apples, then chop them roughly into bite-sized chunks.
Scatter the apple and blackberries into an oven-proof dish. Squeeze the lemon over them, then sprinkle with cinnamon and the sugar.
In a bowl, rub the margarine with the flour and cinnamon until you get breadcrumb-like crumbles, then add sugar, oats and seeds and make lumpy crumble.
Cover the fruits with the crumble evenly, slightly pressing the top down with a spoon.
Bake for 20-25 minutes until golden and crispy.
Serve it warm.