Preheat the oven to 375°F and butter the bottom of a 9×13″ baking dish.
In a glass bowl, mix the flour, oats, sugar, cinnamon, and salt. Whisk well to ensure there are no lumps. Add the cold, cubed butter, and use your hands to rub it in until the mixture looks like large bread crumbs.
In the baking dish, combine the berries, cornstarch, lemon juice, and salt. Mix everything until well combined.
Evenly spread the crumble mixture over the berries.
Bake for 40-45 minutes or until the crumble is golden and crispy.
Serve warm, preferably with vanilla ice cream or cold whipped cream!