01 - Preheat oven to 350°F.
02 - In a large mixing bowl, combine berries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Gently toss until berries are evenly coated.
03 - Pour berry mixture into a lightly greased 8x8 inch baking dish and spread evenly.
04 - In a separate bowl, whisk together rolled oats, all-purpose flour, light brown sugar, ground cinnamon, and salt.
05 - Add cold cubed butter to the oat mixture. Using your fingertips, rub butter into dry ingredients until mixture resembles coarse breadcrumbs.
06 - Evenly scatter oat crumble topping over the berry filling.
07 - Bake in preheated oven for 35 minutes until topping is golden brown and berry filling is bubbling at the edges.
08 - Remove from oven and allow to cool for at least 10 minutes. Serve warm, optionally with vanilla ice cream or custard.