Preheat your oven to 350°F (175°C).
Grease your 9x13-inch baking dish with 1 teaspoon of unsalted butter.
In a large mixing bowl, combine the frozen mixed berries, freshly squeezed lemon juice, ¾ cup granulated sugar, and 2 tablespoons cornstarch. Toss gently until well-coated.
Pour the berry mixture into the prepared baking dish, spreading it out evenly.
In another mixing bowl, whisk together 1 cup all-purpose flour, ½ cup packed dark brown sugar, 1 teaspoon ground cinnamon, and ½ teaspoon salt. Add the cubed cold unsalted butter and mix until crumbly.
Sprinkle the crumble topping evenly over the berry mixture.
Place in the oven and bake for 40-45 minutes, until golden brown and bubbling.
Let it cool for about 10-15 minutes before serving.
Scoop warm Berry Crumble into bowls and top with vanilla ice cream, if desired.
