Preheat the oven to 350 degrees Fahrenheit.
Prepare a 9-inch baking pan by lining it with parchment paper or applying a light coating of baking spray. This aids in preventing sticking and facilitates easier removal of the baked product. Set the pan aside.
In a medium-sized mixing bowl, combine 1/2 cup of brown sugar, the baking powder, a pinch of salt, and 1 1/2 cups of flour. Whisk these ingredients together for approximately 15-30 seconds, ensuring an even mixture.
Add the butter and egg to the dry mixture. Stir the ingredients until they reach a coarse crumb-like texture. You may need to cut the butter in with a pastry blender to help it crumble.
Transfer 2/3 of this mixture into the prepared baking pan. Press down firmly to form an even, compact layer. Use clean hands or the back of a spoon for this process, ensuring uniform thickness.
In another bowl, combine the sliced apples with the remaining 1/4 cup of brown sugar, 2 tablespoons of flour, lemon juice, cinnamon, and nutmeg. Mix until the apples are evenly coated.
Pour the apple mixture over the pressed crust in the pan, smoothing it into an even layer.
Sprinkle the remaining crumb mixture over the apple layer, making sure to cover completely.
Place the pan in the preheated oven and bake for 35-40 minutes, or until the top turns a golden brown color.
Remove the pan from the oven and allow it to cool on a wire rack for 10 minutes. This step is essential for the structural integrity of the bars.
Once cooled, cut the bake into squares. These can be served as they are or with a scoop of ice cream for added indulgence.