01 - Preheat the oven to 350°F. Lightly grease an 8×10-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, toss the mixed berries with granulated sugar, cornstarch, vanilla extract, lemon zest, and lemon juice until evenly coated. Spread the berry mixture in an even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, shredded coconut, light brown sugar, and salt. Add the cold cubed butter and vanilla extract. Using your fingertips or a pastry cutter, rub the butter into the dry ingredients until the mixture forms coarse, pea-sized crumbs.
04 - Scatter the crumble topping evenly over the berry filling, covering the surface as uniformly as possible.
05 - Place the dish on the center oven rack and bake for 30 to 35 minutes, until the topping is deep golden and the berry juices are bubbling around the edges.
06 - Remove from the oven and let cool for 10 to 15 minutes before serving. Serve warm on its own or accompanied by vanilla ice cream or freshly whipped cream.