A moist fruit filling with a crunchy topping.
Crust
1 1/3 cups plus about 3 tablespoons whole-wheat pastry flour
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1 cup packed light brown sugar
2 tablespoons butter, softened
2 tablespoons canola oil
1 large egg
1 teaspoon vanilla extract
Blueberry Filling
1/2 cup granulated sugar
2 tablespoons all-purpose flour
1 teaspoon freshly grated lemon zest
2 cups fresh or frozen blueberries
1 tablespoon lemon juice
Confectioners' sugar for dusting (optional)
1. To make crust: Preheat oven to 350°F. Lightly oil an 8-by-12-inch baking dish or coat it with nonstick cooking spray.
2. Whisk together 1 1/3 cups of the flour, baking powder, baking soda and salt in a large bowl. Beat together brown sugar, butter, oil, egg and vanilla in another large bowl with an electric mixer on high speed until smooth, making sure no lumps of brown sugar remain, about 1 minute. Add the dry ingredients and stir with a wooden spoon until well blended. (The dough will be quite firm.)
3. Transfer two-thirds of the dough to the prepared baking dish; cover the dough with a piece of plastic wrap and use it to press the dough into the bottom of the dish in an even layer. Remove the plastic wrap. Bake until puffed and golden, about 15 minutes.
4. To make the topping: Gradually work enough of the remaining 3 tablespoons flour into the remaining dough using your fingertips, until it resembles coarse crumbs.
5. To make filling: Stir together sugar, flour and lemon zest in a small bowl.
6. Combine blueberries and lemon juice in a medium saucepan; cook, stirring, over medium heat until the berries begin to exude juice. Add the sugar mixture and stir until the filling reaches a simmer and thickens.
7. Push down the higher outside edges of the baked crust with a wooden spoon; pour the hot filling over it and spread all the way to the sides of the dish. Sprinkle the crumb topping over the top. Bake until the topping is golden, 15 to 20 minutes longer.
8. Transfer the baking dish to a rack and let cool, covered with a kitchen towel to soften the crumbs slightly. Cut into 15 bars. Dust lightly with confectioners' sugar if using. Store at room temperature in an airtight container.