For the crumble topping
120g plain flour
60g castor sugar
60g unsalted butter at room temperature, cut into pieces
For the fruit
30g brown sugar
¼ tsp ground cinnamon
Combine the flour and sugar into a large bowl.
Add the butter and rub into the flour using your fingertips to make a light breadcrumb texture.
Sprinkle the mixture evenly over a baking sheet and bake for 15 mins at 190°C.
Peel, core and cut the apples into 2cm pieces.
Put the butter and sugar in a medium saucepan and melt together over a medium heat.
Cook for 3 mins until the mixture turns to a light caramel.
Stir in the apples and cook for 3 mins.
Add the blackberries and cinnamon, and cook for 3 mins more.
Cover, remove from the heat, and leave for 2-3 mins to continue cooking in the warmth of the pan.
To serve, spoon the warm fruit into an ovenproof gratin dish, top with the crumble mix, then reheat in the oven at 190°C for 5-10 mins.