•3 cups fresh or frozen blueberries
•1 tablespoon lemon juice
•1 teaspoon lemon zest
•1/2 cup sugar
•2 tablespoon cornstarch
For the Crumble and Crust:
•1 1/2 cups all purpose flour
•1 1/2 cups old fashioned rolled oats
•1 cup brown sugar
•1 ½ teaspoons lemon zest
•3/4 cup butter, softened and cubed
•1/2 teaspoon baking soda
•1/2 teaspoon salt
•1 teaspoon vanilla extract
1.Preheat the oven to 350°F. Butter the bottom and sides of a 9-by-13-inch baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
2.In a medium saucepan over medium heat, cook blueberries, lemon juice and lemon zest until fruit is tender, about 8-10 minutes.
3.In a small bowl, whisk together sugar and cornstarch. Stir into fruit mixture and continue cooking until mixture comes to a boil. Boil for 1-2 minutes, until thick. Scrape berry filling into a medium bowl and set aside.
4.In the bowl of a food processor fitted with the metal blade, pulse together flour, oats, brown sugar, butter, baking soda, salt and vanilla extract until thoroughly blended and resembles coarse crumbs.
5.Reserve 1 ½ cups of the mixture and set aside. Pour the rest of the mixture into the prepared pan and use your hands or the back of a large wooden spoon to push the crust into an even layer at the bottom of the pan.
6.Spread the blueberry filling evenly on top of the crust. Sprinkle the reserved 1 1/2 cup crust mixture evenly on top of the filling.
7.Bake for 30 to 35 minutes, rotating the pan every 15 minutes, until the top is golden brown and the filling starts to bubble around the edges.
8.Transfer to a wire rack to cool completely, then cut into squares and serve. The bars can be stored in the refrigerator in an airtight container for up to two days.