Blueberry season is here and will soon be gone. I put 20 pounds of berries into the freezer this week with the help of my mom and dad, and my husband. They are undoubtedly best fresh, preferably eaten off the bushes in the back yard, but these bars definitely qualify for runners-up. The recipe is from Fine Cooking magazine, and I haven't done a thing to it other than make it several times. (edited to add: as I typed the recipe, I realized that I have made a few minor changes.)
1 cup butter, softened
3 cups flour
1 1/2 cups old-fashioned rolled oats
1 1/3 cups light brown sugar (I used a bit less)
1 tsp table salt
1 tsp baking powder
1 large egg, separated
14 oz can sweetened condensed milk (I used the light version)
1/2 cup fresh lemon juice
2 tsp grated lemon zest
2 1/2 cups blueberries, washed and drained (I've made this with equal success using frozen blueberries, slightly thawed)
Preheat the oven to 350 degrees. Grease a 9 x 13 glass baking pan.
In a large bowl, combine the flour, oats, sugar, salt and baking powder. Using your fingers, blend the butter completely into the flour mixture. Transfer 2 1/2 cups to another bowl and reserve that for the topping.
Blend the egg white with the remaining crumb mixture and press into the bottom of the pan to form a level crust. Bake the crust until it starts to form a dry top, 10 to 12 minutes.
Meanwhile, in a medium bowl, whisk together the condensed milk, lemon juice, lemon zest and egg yok. Let this mixture stand for 5 minutes; it will be begin to thicken.
Sprinkle the blueberries evenly over the hot crust and then drop spoonfuls of the lemon mixture over the berries. Spread gently to make it more or less even, without becoming too concerned since it all levels out in the cooking. Bake until the lemon mixture just begins to form a shiny skin, 7 to 8 minutes (a wee bit longer if you use frozen berries).
Sprinkle the reserved topping mixture over the lemon-blueberry layer. Bake until the filling is bubbly at the edges and the topping is golden brown, 25-30 minutes.
Let cool, then cut into bars. I like this warm. The bars freeze well, and heated up just slightly in the microwave makes for a bit of summertime all year round. Keep in the refrigerator if you don't freeze them.