Preheat the oven to 200C. Pour the lemon juice over the apples - this will add flavour and stops the apples from going brown. Layer the apples, blackberries, and sugar in a lasagne dish. Place the flour in a large bowl and then rub in the butter until it resembles breadcrumbs. Add the muesli and another 50g sugar and mix through. Sprinkle the crumble topping over the fruit. Bake for 45 minutes. Cool for a few minutes and then serve with custard or cream.
