01 - Preheat the oven to 350°F. Lightly grease a 9-inch square or similar-sized baking dish with butter or nonstick spray.
02 - In a large mixing bowl, gently toss the mixed berries with maple syrup, cornstarch, vanilla extract, lemon juice, and a pinch of salt until evenly coated. Transfer the mixture to the prepared baking dish and spread into an even layer.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, brown sugar, ground cinnamon, and salt. Add the cold cubed butter and cut it into the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs. Drizzle in the maple syrup and stir just until evenly distributed.
04 - Scatter the oat topping evenly over the berry filling, covering the surface completely.
05 - Bake for 35 to 40 minutes, or until the topping is deep golden brown and the berry filling is bubbling vigorously around the edges of the dish.
06 - Let the crisp rest for at least 10 minutes before serving to allow the filling to set. Serve warm, optionally accompanied by vanilla ice cream or freshly whipped cream.