Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment paper for easy removal.

Make the Crust and Topping:
In a large bowl, combine the flour, sugar, baking powder, and salt.
Cut in the cold cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Beat in the egg until the dough begins to come together, but still a bit crumbly.
Reserve about 1 cup of the mixture for the topping, then press the remaining dough evenly into the bottom of the prepared pan to form the crust.

Make the Apple Filling:
In a separate bowl, toss the diced apples with lemon juice to prevent browning.
Stir in the sugar, cinnamon, flour, and nutmeg (if using) until the apples are evenly coated.

Assemble the Bars:
Spread the apple mixture evenly over the crust.
Sprinkle the reserved crumb mixture over the top of the apples to form a crumbly topping.

Bake:
Bake the bars in the preheated oven for 45-50 minutes, or until the top is golden brown and the apple filling is bubbling.

Cool and Slice:
Allow the bars to cool completely in the pan before slicing into squares or bars. They’re easier to cut once they’ve cooled, and the flavors will have had time to set.

Serve and Enjoy:
Serve these warm or at room temperature. You can dust them with powdered sugar or serve with whipped cream or vanilla ice cream for an extra treat!