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 bowl, combine the mixed berries, maple syrup, cornstarch, vanilla extract, lemon juice, and a pinch of salt. Gently toss until all the berries are evenly coated. Pour the mixture into the prepared baking dish and spread into an even layer.
03 - In a separate bowl, whisk together the oats, flour, brown sugar, cinnamon, and salt. Add the cold butter cubes and use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Fold in the chopped nuts if using.
04 - Sprinkle the crisp topping evenly over the berry filling, covering the surface as uniformly as possible.
05 - Bake for 35 to 40 minutes, or until the topping is deep golden brown and the berry juices are bubbling around the edges of the dish.
06 - Allow the crisp to cool for at least 10 minutes before serving. Serve warm, optionally topped with vanilla ice cream or fresh whipped cream.