Preheat your oven to 350°F (175°C).
Begin by preparing the berry filling.
In a mixing bowl, combine the mixed berries, erythritol (or another low-carb sweetener), lemon zest, and lemon juice.
Mix until the berries are well-coated.
Transfer the berry mixture into a baking dish, spreading them out evenly.
Now, for the crumble topping.
In a separate mixing bowl, whisk together almond flour, shredded coconut, erythritol, vanilla extract, and salt.
Add the cold, cubed butter to the dry mix.
Using your fingers or a pastry cutter, work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Evenly sprinkle the crumble mixture over the berries in the baking dish.
Place the baking dish in the oven and bake for 20 minutes or until the crumble topping turns a golden brown.
Once baked, remove from the oven and allow to cool slightly before serving.