Preheat the oven to 190C/375F/gas mark 5. Grease a 20cm/8in square tin and line with baking parchment.
Mix together the oats, flour, baking powder, sugar, cinnamon and salt in a large bowl.
Add the vegan butter or coconut oil and rub in using your fingertips until the mixture resembles breadcrumbs.
Tip about two thirds of the mixture into the prepared tin. Spread it out level and pack it down very firmly.
Bake for about 15 minutes until golden around the edges.
To make the filling, mix the cornflour and lemon juice together in a large bowl to make a paste. Stir in the jam followed by the berries.
Tip the filling over the base and spread level.
Scatter over the remaining crumble mixture. Bake for 25-30 minutes until the crumble is golden and the filling is bubbling.
Leave to cool in the tin then carefully lift it out using the baking parchment, cut into squares and serve.
Store any leftovers in the fridge.