Preheat the oven to 200C/400F/Gas 6.
Peel, core and chop the apples into small chunks. Cut the lemons in half and squeeze the juice over the apple and mix well. This not only adds flavour but prevents the freshly peeled apples from discolouring.
Layer the apples, blackberries, and sugar in a large pie dish (a lasagne dish is ideal).
Place the flour in a large bowl and then rub in the butter until it resembles breadcrumbs - leave a few lumps of butter so that the topping is not too fine. Add the oats and the sugar and mix through.
Use a spoon to sprinkle the crumble topping evenly over the fruit. Bake for 45 minutes or until the fruit is cooked and bubbling juices seep through the topping.
Cool for a few minutes and then serve with fresh cream.
For the filling
450g bramley apples
85g demerara sugar
For the topping
110g plain flour
30g demerara sugar