01 - Preheat oven to 350°F. Lightly grease a 9-inch square or round baking dish.
02 - In a large mixing bowl, gently combine mixed berries with cornstarch, maple syrup, vanilla extract, lemon juice, and a pinch of salt until evenly coated. Transfer the mixture to the prepared baking dish and spread evenly.
03 - In a separate bowl, mix oats, flour, nuts if using, cinnamon, and salt. Stir in maple syrup, melted butter or coconut oil, and vanilla extract until the mixture forms a crumbly consistency.
04 - Evenly sprinkle the oat topping over the berry filling to cover the surface.
05 - Place the baking dish in the oven and bake for 35 to 40 minutes, until the topping is golden brown and the berry filling is bubbling.
06 - Allow the crisp to cool slightly before serving. Serve warm, optionally topped with vanilla ice cream or whipped cream.