Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the frozen mixed berries with the freshly squeezed lemon juice. Add the granulated sugar and cornstarch, stirring gently to coat the berries.
Pour the berry mixture into your prepared 9x13 baking dish, spreading it out evenly.
In a separate bowl, mix together the all-purpose flour, dark brown sugar, ground cinnamon, and salt. Add the cold cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the berry mixture in the baking dish.
Place the baking dish in the preheated oven and bake for 35-40 minutes, or until the topping is golden brown and the berries are bubbling.
Once baked, remove the berry crumble from the oven and let it cool for about 10-15 minutes before serving.
