01 - Set oven to 350°F. Thoroughly grease a 9-inch baking dish to prevent sticking.
02 - Gently toss mixed berries with maple syrup, cornstarch, vanilla extract, lemon juice, and a pinch of salt in a large mixing bowl until evenly coated. Transfer this mixture to the prepared baking dish and spread in an even layer.
03 - In a separate bowl, combine rolled oats, all-purpose flour, light brown sugar, ground cinnamon, and salt. Add cold, cubed butter and blend it in using fingertips or a pastry cutter until the mixture forms coarse crumbs. Stir in chopped pecans or walnuts if desired.
04 - Evenly sprinkle the crumble mixture over the prepared berry filling covering the surface completely.
05 - Place dish in the oven and bake for 35 minutes until the topping becomes golden brown and the berry filling is bubbling.
06 - Remove from the oven and allow to cool for 10 minutes before serving. Enjoy warm, with optional toppings such as vanilla ice cream or whipped cream.