Shortbread Base:
1 1/2 sticks (3/4 cup) unsalted butter, cut into
1/2-inch pieces
2 cups all-purpose flour
1/2 cup packed light brown sugar
1/2 tsp. kosher salt
Lemon Filling:
4 large eggs
1 1/2 cups granulated sugar
3/4 cup fresh lemon juice
1/3 cup all-purpose flour
1 cup berries
Heat oven to 350F. Line a 9x13x-2-inch baking pan with foil. In a food processor, pulse all the shortbread ingredients until mixture begins to form small lumps. Sprinkle mixture into prepared pan and press evenly onto bottom. Bake until golden, about 20 minutes.
Meanwhile, make filling. In a large bowl whisk together eggs and sugar, then stir in lemon juice and flour. Pour lemon mixture over shortbread and sprinkle with berries. Reduce oven temperature to 325°F and bake until set, about 30 minutes. Let cool completely in pan. Lift foil to remove from pan, then cut into bars.