Preheat your oven to 375°F (190°C). Lightly grease a 9x9-inch baking dish or a deep pie dish with a little butter or cooking spray to prevent the fruit from sticking.

In a large mixing bowl, combine the chopped apples, blackberries, granulated sugar, lemon juice, and half of the ground cinnamon (1/2 teaspoon). Toss the mixture gently until the fruit is evenly coated, then spread it into an even layer in your prepared baking dish.

To make the crumble topping, whisk together the all-purpose flour, rolled oats, brown sugar, remaining 1/2 teaspoon of cinnamon, and salt in a separate medium bowl. Ensure there are no large lumps of brown sugar left in the dry mix.

Add the cold, cubed butter to the dry ingredients. Using your fingertips or a pastry blender, rub the butter into the flour mixture until it resembles coarse breadcrumbs, leaving a few larger pea-sized clumps for a rustic texture.

Sprinkle the crumble mixture evenly over the fruit in the baking dish, being careful not to pack it down tightly to ensure a crisp finish. Bake in the preheated oven for 40 to 45 minutes, or until the topping is golden brown and the fruit juices are bubbling visibly around the edges.

Remove the crumble from the oven and allow it to rest for 10 minutes so the fruit filling can slightly thicken. Scoop into warm bowls and serve immediately, ideally topped with hot vanilla custard or a generous scoop of cold vanilla bean ice cream.