Preheat your oven to 350°F (175°C).

In a large bowl, combine mixed berries, lemon juice, and cornstarch. Toss gently to coat.

Pour the berry mixture into a greased 9x9-inch baking dish, spreading it evenly across the bottom.

In another bowl, mix together the rolled oats, flour, brown sugar, cinnamon, and salt until well combined.

Add the melted butter to the oat mixture and stir until crumbs form.

Sprinkle the crumble mixture evenly over the berries in the baking dish.

Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the berries are bubbling.

Allow to cool slightly before serving warm with a scoop of vanilla ice cream or whipped cream.