Preheat your oven to 350°F (175°C). Lightly grease a 9×9-inch baking dish.

In a large bowl, combine the raspberries, blackberries, blueberries, granulated sugar, cornstarch, lemon juice, lemon zest, and salt. Toss gently to coat the fruit.

Transfer the berry mixture to the prepared baking dish, spreading it into an even layer.

In another bowl, stir together the oats, flour, brown sugar, cinnamon, and salt. Add the cold butter and use a pastry cutter or fork to blend until the mixture resembles coarse crumbs.

Sprinkle the oat crumble evenly over the berries.

Bake for 30 minutes, or until the topping is golden brown and the fruit is bubbling at the edges.

Let the crisp rest for 10 minutes before serving. Enjoy warm with a scoop of vanilla ice cream or a dollop of whipped cream.