Preheat oven to 400 degrees. Spray an 8x8-inch baking dish with non-stick spray.
In a medium bowl combine 1/4 cup brown sugar, all-purpose flour, whole-wheat pastry flour, baking powder, cinnamon, and lemon zest. Cut in butter using a pastry blender or fork until the butter is the size of peas and the mixture resembles coarse meal. Add the egg and mix just until blended and moistened. Press two-thirds of dough into prepared pan, reserving remaining dough, and bake for 5 minutes. Let cool 10 minutes. Reduce oven temperature to 375 degrees.
In another medium bowl combine 1/4 cup brown sugar, chopped peaches, and 1 tablespoon flour. Pour peach mixture evenly over cooled crust. Crumble reserved dough mixture over peaches. Bake for 30 minutes, or until top is lightly browned. Cool before cutting into 9 large squares.