Fill one of the mixing bowls with cold water and add the juice of the lemon
Use a vegetable peeler to peel the apples easily.
Cut each apple into quarters and remove the core.
Chop each quarter into chunks - about 9 chunks per quarter
As you chop each quarter, place the chunks of apple into the lemon water. this prevents the pieces turning brown while you continue to peel and chop.
Drain the apples and place into the frying pan with half of the sugar and a small splash of water - about a tablespoon
Heat the apples, stirring together with the sugar, until they start to soften.
Taste at this point to see if the apples are sweet enough. Add more sugar if necessary and cook for another minute and taste again.
When the apples are sweet enough add in the blackberries.
Continue to cook over a low heat until the blackberries start to release their juice and turn the apples a beautiful ruby red.
Take the frying pan off the heat immediately and spoon the fruit into the ovenproof baking dish
Now make the crumble
Place the flour into the other mixing bowl and rub in the butter gently with just you fingertips until the mixture looks like wet sand.
Stop at this stage and stir in the caster sugar
Continue to work the mixture very gently until it starts to form small clumps then stop.
Spoon the crumble mixture ontop of the fruit but do not press it down - this is a rich, buttery mixture that will form a crust if you do!
Gently spread the mixture evenly so that it reaches all the edges
Sprinkle over the demerara sugar for extra crunch
Bake for approx. 30 minutes until the crumble is beautifully golden
Preheat oven to 190℃ / 170℃ Fan / 375℉ / Gas 5