Step 1

Preheat the oven to 350°F. Line a 9x13-inch metal baking pan with foil, leaving a 1-inch overhang on the ends. Spray the foil with cooking spray- bottom and sides of the pan.

Step 2

In a large bowl, whisk together the flour, oats, sugar, salt, and baking powder. Using a pastry cutter (or your fingers), blend the butter completely into the flour mixture. Transfer 2 cups of the crumb mixture to another bowl and reserve for the topping.

Step 3

Blend the egg white into the remaining crumbs and then press the mixture into the bottom of the pan to form a level crust. Use the bottom of a flat, wide glass to tap the mixture and even it out.

Step 4

Bake the crust 10 to 12 minutes, or until it starts to form a dry top.

Step 5

Meanwhile, in a medium bowl, whisk the condensed milk, lemon juice & zest, and egg yolk. Let the mixture stand for 5 minutes (it will begin to thicken).

Step 6

Sprinkle the blueberries evenly over the hot crust and then drop spoonfuls of the lemon mixture over the blueberries. Spread gently with a spatula to distribute as evenly as you can. Bake until the lemon mixture begins to form a shiny skin- 7 to 8 minutes.

Step 7

Sprinkle the reserved crumble topping over the lemon-blueberry layer, pressing the streusel between your fingers into small lumps as you sprinkle. Bake until the filling is bubbly at the edges and the topping is brown, 25 to 30 minutes.

Step 8

Let the bars cool in the pan on a rack until just warm, about an hour. Carefully lift them out of the pan using the foil overhang and transfer to a wire rack to cool completely. Remove the foil and cut into 24 bars when cool. (If you have time to chill the bars, they're easier to cut cleanly when chilled).