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. Butter the parchment.
For the Crumble: Combine flour, sugar, baking powder, salt, and cinnamon. 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. Put half of the dough into the prepared pan. Place the pan and the remainder of the dough in the refrigerator while you prepare the filling. For the Peach Filling: Gently toss diced peaches and lemon juice together; set aside.In a separate 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 a preheated oven for 45 minutes, or until the top is slightly brown. Cool completely on a wire rack then transfer to the refrigerator to chill before cutting into squares.