Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking pan with cooking spray or butter.

In a large mixing bowl, combine all-purpose flour, granulated sugar, light brown sugar, baking powder, cinnamon, and salt. Whisk until fully blended.

Cut in the cold unsalted butter until the mixture resembles coarse crumbs. Blend in the beaten egg until the dough comes together.

Reserve about 1½ cups of the crumb mixture and press the remaining dough into the bottom of the prepared baking pan. Bake for 15 minutes.

Slice the apples and toss them with lemon juice. In a separate bowl, mix together flour, cinnamon, and nutmeg.

Toss the apple slices with the spice mixture and melted unsalted butter, then spread over the baked crust.

Sprinkle the reserved crumb mixture over the apples and bake for an additional 30-35 minutes, until golden brown.

Remove from the oven and let cool completely in the pan before cutting.