01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large bowl, whisk together the flour, rolled oats, brown sugar, baking powder, and salt. Pour in the melted butter and stir until the mixture is evenly moistened and crumbly in texture.
03 - Set aside 1 cup of the crumble mixture for the topping. Transfer the remaining mixture to the prepared pan and press firmly into an even, compact layer across the bottom.
04 - Bake the base for 10 minutes until just set. Remove from the oven and allow to cool slightly while preparing the filling layers.
05 - In a high-speed blender, combine the soaked and drained cashews, coconut milk, lemon juice, lemon zest, granulated sugar, maple syrup, vanilla extract, and a pinch of salt. Blend on high until completely smooth and creamy, scraping down the sides as needed.
06 - In a medium bowl, gently toss the blueberries with granulated sugar, cornstarch, and lemon juice until the berries are evenly coated.
07 - Pour the cashew lemon cream over the par-baked base and spread into an even layer. Scatter the blueberry mixture evenly across the cream surface.
08 - Sprinkle the reserved crumble mixture evenly over the blueberries. Bake for 25 to 28 minutes until the top is golden brown and the filling is just set.
09 - Cool completely in the pan at room temperature, then refrigerate for at least 2 hours. Use the parchment overhang to lift the slab out, then slice into 12 bars.