Place a rack in the center of your oven and heat toÂ 375 degrees F. Line anÂ 8x8-inch baking pan with parchment paper so that the paper overhangs two sides like handles.
In a medium bowl, combine the oats, white whole wheat flour, brown sugar, cinnamon, and salt. Pour in the melted butter and stir until it forms clumps and the dry ingredients are evenly moistened. Set aside 1/2 cup of the crumble mixture, then press the rest into an even layer in the bottom of the prepared pan.
ScatterÂ half of the blueberries over the crust. Sprinkle the cornstarch evenly over the top, then sprinkle on the lemon juice and 1/2 tablespoon granulated sugar. Scatter on the remaining berries, then the remaining 1/2 tablespoon sugar. SprinkleÂ theÂ reserved crumbs evenly over the top. You will have some fruit showing through.
Bake the bars for 30 to 40Â minutes, until the fruit is bubbly and the crumb topping smells toasty and looks golden. Place the pan on a wire rack to coolÂ completelyÂ (you can speed this process along in the refrigerator).
While the bars cool, prepare the glaze: in a medium bowl, briskly whisk together the powdered sugar, vanilla, and milk until smooth. Feel free to add more milk if a thinner consistency is desired. Using the parchment paper handles, lift the bars from the pan. Drizzle with the glaze, slice, and serve.