01 - Set oven temperature to 350°F (180°C) and prepare for baking.
02 - Combine the winter berries, granulated sugar, cornstarch, lemon juice, and vanilla extract in a large bowl. Toss gently until berries are evenly coated, then transfer to a lightly greased 9-inch baking dish.
03 - In a separate bowl, blend rolled oats, all-purpose flour, light brown sugar, cinnamon, and salt. Incorporate cold cubed butter by rubbing it with your fingertips until the mixture resembles coarse crumbs.
04 - Evenly distribute the oat crumble mixture over the berry filling in the baking dish.
05 - Place the baking dish in the oven and bake for 35 minutes, or until the topping is golden brown and the berry filling bubbles around the edges.
06 - Allow the crumble to cool for 10 minutes before serving warm. Optionally accompany with vanilla ice cream or whipped cream.