01 - Preheat the oven to 350°F (175°C). Lightly grease a 2-quart baking dish with butter or cooking spray.
02 - In a large bowl, combine the mixed berries, maple syrup, cornstarch, vanilla extract, and lemon juice. Add a pinch of salt and toss gently until the fruit is evenly coated.
03 - Pour the coated berry mixture into the prepared baking dish, spreading it into an even layer.
04 - In a separate medium bowl, whisk together the oats, flour, chopped nuts (if using), brown sugar, cinnamon, and salt.
05 - Pour the melted butter, vanilla extract, and maple syrup into the dry oat mixture. Stir until evenly moistened and crumbly.
06 - Sprinkle the oat mixture evenly over the berries in the baking dish.
07 - Bake for 35 minutes, or until the topping is golden brown and the fruit filling is bubbling around the edges.
08 - Remove from the oven and let cool for at least 10 minutes before serving. Serve warm.