from The Kitchen Sink Recipes (who adapted the recipe from the Byerly’s/Lund’s Magazine)
2 cups old-fashioned rolled oats
2 cups white whole wheat flour
1 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon kosher salt
1/2 teaspoon ground cinnamon
6 tablespoons unsalted butter, chilled
1 cup milk (I used 1%)
1 tablespoon freshly squeezed lemon juice
2 teaspoons vanilla extract
1/2 cup sugar
1 tablespoon cornstarch
2 1/2 cup frozen blueberries
2 tablespoons brown sugar
Preheat oven to 350 F. Spray a 9-by-13-inch baking pan with cooking spray, line with a piece of parchment (allowing the sides to hang over the pan) and then spray the parchment with more cooking spray.
In a medium bowl, whisk together the oats, flour, baking powder, baking soda, salt and cinnamon. Using the large holes of a box grater, shred the butter into the flour and oat mixture. Using a pastry cutter, work the butter into the mixture until it resembles pebbles or rice.
In a measuring cup, whisk together the milk, lemon juice and vanilla. Pour the milk mixture over the dry ingredients and, using a fork, stir until everything is just combined.
In the bowl of a food processor, pulse the sugar and cornstarch several times. Add 1 cup of the blueberries and pulse until the berries are minced. Transfer the mixture to a bowl and stir in the remaining whole blueberries.
Press half of the oat mixture into the bottom of the prepared baking pan to create an even layer. I found it easiest to do this by placing a sheet of plastic wrap on top of the oat mixture; otherwise, the oat mixture stuck to my fingers too much. Spread the blueberry mixture evenly over the oat crust. Dollop the remaining oat mixture on top of the blueberry mixture, distributing it evenly across the entire pan. Sprinkle with the brown sugar.
Bake 50 to 55 minutes, or until the blueberry mixture is bubbling and the oat mixture is golden brown. Transfer the pan to a cooling rack and allow the bars to cool completely in the pan. Remove and slice into square- or rectangle-shaped bars.