01 - Set oven temperature to 350°F.
02 - In a large bowl, combine mixed berries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Gently toss until evenly coated. Transfer to a lightly greased 9-inch baking dish.
03 - In a separate bowl, stir together rolled oats, all-purpose flour, light brown sugar, cinnamon, and salt. Add cold, cubed butter and rub with fingertips until mixture forms coarse crumbs.
04 - Evenly distribute oat crumble topping over berry mixture in the baking dish.
05 - Bake for 35 minutes or until the topping is golden and berry filling bubbles at edges.
06 - Allow to cool for 10 minutes before serving. Best enjoyed warm, optionally accompanied by vanilla ice cream or whipped cream.