Preheat oven to 375 °F (185 °C). Spray a 9x13 inch baking dish with nonstick cooking spray; set aside.
Make crumble and Crust: In a medium bowl, combine 3 cups of flour, 1 cup of sugar, baking powder (1 teaspoon), and salt (1/4 teaspoon). Add the cold butter (1 cup) and toss with the flour mixture. Using a pastry blender or two knives, cut the butter into the flour mixture, until it's the size of peas.
In a small mixing bowl whisk together egg (1) and vanilla (1 teaspoon) then stir into the crumb mixture. The dough will be crumbly. Press half of the dough into the prepared baking pan. Reserve the other half.
Make Filling: In another bowl, stir together 1/2 cup sugar, cinnamon (1/2 teaspoon), nutmeg (1/4 teaspoon), lemon juice (1 tablespoon), and cornstarch (4 teaspoons). Add the sliced peaches and gently mix. Spoon the peaches (4 cups) over the crust. Crumble the remaining dough over the peach mixture.
Bake for 45-55 minutes, until the top is slightly brown. Cool completely before cutting into squares. Stores really well in the fridge.