01 - Set the oven temperature to 350°F (180°C) and allow it to fully preheat.
02 - In a large bowl, combine the berries, granulated sugar, cornstarch, orange zest, and orange juice; toss gently to coat evenly.
03 - Transfer the berry mixture to a 8x8-inch baking dish and spread into an even layer.
04 - In a separate bowl, whisk together flour, brown sugar, rolled oats, chopped nuts if using, ground cinnamon, and salt.
05 - Add cold butter cubes to the dry mixture and rub with fingertips until coarse crumbs form.
06 - Evenly sprinkle the crumble topping over the berry filling in the baking dish.
07 - Bake for 35 minutes, or until the topping is golden brown and the berry filling bubbles.
08 - Remove from oven and let rest for 10 minutes before serving.