Preheat your oven to 375°F (190°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper with an overhang.

In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, cinnamon, and salt.

Add the cold, cubed butter. Use a pastry cutter or your fingers to cut the butter in until the mixture resembles coarse crumbs.

Stir in the lightly beaten egg until just combined. Reserve 1 ½ cups of the mixture for the topping. Press the rest firmly into the bottom of the prepared pan.

Bake the crust for 15 minutes until lightly golden at the edges.

While the crust bakes, combine sliced apples and lemon juice in a large bowl. In a separate small bowl, whisk together flour, cinnamon, and nutmeg. Add the spice mixture and melted butter to the apples and toss to combine.

Spread the apple filling evenly over the warm, partially baked crust. Sprinkle the reserved 1 ½ cups of crumb mixture evenly over the apple layer.

Bake for 30-35 minutes, until the topping is golden brown and the filling is bubbly.

Let the bars cool completely in the pan on a wire rack before cutting into squares. This is essential for clean cuts.