Preheat the oven to 350°F (175°C). In a mixing bowl, combine the mixed berries, sugar, lemon juice, and cornstarch. Toss gently to coat the fruit evenly.

Pour the fruit mixture into a greased baking dish and spread it out evenly.

In another mixing bowl, whisk together the flour, baking powder, sugar, and salt.

Cut in the cold butter until the mixture resembles coarse crumbs.

Stir in the milk until just combined. Do not overmix.

Drop spoonfuls of the biscuit topping over the fruit filling, leaving some gaps for the fruit to show through.

Bake in the preheated oven for 30 minutes or until the topping is golden brown and the fruit is bubbly.

Remove from the oven and let cool slightly before serving.