Preheat oven to 350°F (175°C) and lightly grease an 8x8 or 9x9 inch baking dish.
In a large mixing bowl, toss the berries with lemon juice, 2 tablespoons of maple syrup, and cornstarch until the fruit is well coated.
Transfer the berry mixture into the prepared baking dish and spread evenly.
In a separate bowl, combine the oats, almond flour, chopped walnuts, cinnamon, and sea salt.
Pour the melted coconut oil and remaining 1/4 cup of maple syrup over the oat mixture and stir until a crumbly texture forms.
Sprinkle the crumble mixture evenly over the berry base.
Bake for 35 to 40 minutes, or until the fruit juices are bubbling and the topping is golden brown.
Remove from the oven and let cool for 10 minutes to allow the juices to thicken before serving.