01 - Set your oven to 350°F (180°C) to warm up while preparing the components.
02 - In a large bowl, gently toss together the berries, granulated sugar, cornstarch, orange zest, and orange juice until evenly coated.
03 - Evenly spread the berry mixture into a 8x8 inch (20x20 cm) baking dish.
04 - In a separate bowl, combine flour, brown sugar, rolled oats, chopped nuts if using, cinnamon, and salt.
05 - Add the cold cubed butter to the dry ingredients and rub together with your fingertips until the mixture achieves a coarse breadcrumb texture.
06 - Evenly distribute the crumble topping over the berry filling in the baking dish.
07 - Place the dish in the preheated oven and bake for 35 minutes, or until the topping is golden brown and the fruit filling is bubbling.
08 - Allow the dessert to cool for 10 minutes prior to serving to enhance flavors and texture.