01 - Preheat the oven to 350°F.
02 - In a large bowl, combine the mixed berries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until the berries are evenly coated.
03 - Transfer the berry mixture to a greased 9-inch baking dish, spreading evenly.
04 - In a separate bowl, mix the flour, oats, brown sugar, cinnamon, and salt. Add the diced cold butter and rub it in with your fingertips until the mixture resembles coarse crumbs.
05 - Sprinkle the oat crumble topping evenly over the berries.
06 - Bake for 35 minutes, or until the topping is golden brown and the fruit filling is bubbling.
07 - Allow to cool for 10 minutes before serving.