Preheat the oven to 375 degrees F. Butter the bottom and sides of a 9-by-13-inch glass or light-colored metal 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.
For the Crumble: In the bowl of a stand mixer fitted with the paddle attachment combine flour, sugar, baking powder, salt and cinnamon on low speed. Add the butter, toss with a fork to coat with the flour mixture. Add egg and mix on medium-low speed until the texture resembles coarse cornmeal, with the butter pieces no larger than small peas. Pat half of dough into the prepared pan. Place reserved half aside. Place the pan and the remainder of the dough in the refrigerator while you prepare the filling.
For the Peach Filling: In medium bowl, gently toss diced peaches and lemon juice together; set aside.
In a separate small bowl, whisk together the flour, sugar, salt, cinnamon and nutmeg. Pour over the peaches and mix gently.
Spread the peach mixture evenly over the crust. Crumble the remaining dough over the peach layer.
Bake in preheated oven for 45 minutes, or until top is slightly brown. Cool completely on wire rack then transfer to the refrigerator to chill before cutting into squares. Store in an airtight container in the refrigerator.