01 - Set oven to 350°F for baking.
02 - Combine frozen berries, granulated sugar, cornstarch, lemon juice, and vanilla extract in a large bowl. Toss gently until berries are evenly coated with the mixture.
03 - Pour the berry mixture into a greased 9-inch baking dish and spread into an even layer.
04 - Whisk together flour, oats, brown sugar, cinnamon, and salt in a separate bowl. Add cold diced butter and work it into the dry mixture with your fingertips until it resembles coarse crumbs.
05 - Distribute the oat crumble topping evenly over the berry filling.
06 - Bake for 35 minutes until the topping turns golden brown and the fruit filling bubbles at the edges.
07 - Let the crumble cool for 10 minutes before serving warm.