Preheat your oven to 375°F. Lightly grease an 8x8-inch baking dish.

In a medium bowl, combine the flour, granulated sugar, brown sugar, salt, and cinnamon for the crumble. Pour in the melted vegan butter and stir with a fork until the mixture forms coarse crumbs. Set aside.

In a large bowl, whisk together the 3 tablespoons sugar, 1/4 cup flour, cornstarch, and the zest from the Meyer lemon.

Add the frozen blackberries to the flour mixture and toss gently until the berries are evenly coated. Squeeze in the lemon juice and stir to combine.

Transfer the berry mixture to the prepared baking dish. The berries will mound up high; try to spread them evenly, piling them slightly higher in the center. Press down gently to pack them.

Sprinkle the crumble topping evenly over the blackberries. If necessary, carefully place some crumble pieces to avoid spilling over the edge.

Bake for 45 minutes, or until the topping is golden brown and the filling is bubbling. Let the crumble cake rest for 20 minutes to allow the juices to thicken. Serve warm or at room temperature.