Preheat the Oven: Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
Make the Dough: In a large bowl, whisk together flour, granulated sugar, baking powder, baking soda, and salt. Add the softened butter and eggs, mixing until a dough forms.
Create the Base Layer: Press 2/3 of the dough into the bottom of the prepared baking pan to form an even layer.
Prepare the Apples: In another bowl, toss the sliced apples with lemon juice, brown sugar, cinnamon, and nutmeg until evenly coated.
Layer the Apples: Spread the apple mixture over the base layer in the baking pan.
Add Crumb Topping: Crumble the remaining dough over the apple filling evenly. Sprinkle oats and walnuts over the top for added crunch if desired.
Bake: Bake in the preheated oven for approximately 40-45 minutes, or until the top is golden brown.
Cool and Serve: Let the bars cool in the pan before cutting into squares. Enjoy!