Peel and core the apples and cut into a 1/2-inch dice.

Melt the butter in a large sauté pan over medium heat. Add the apples and cook until they are soft, about 8 minutes.

Add the sugar, lemon juice, flour and cinnamon to the apples and stir to combine.

Cook until the mixture starts to bubble, then turn the heat to low and cook, stirring constantly, for 3 minutes.

Remove to a bowl to cool while you prepare the crust.

Line a 13x9-inch baking dish with foil or parchment paper and lightly spray with nonstick cooking spray.

In a bowl, whisk together the flour, baking powder and salt.

In a stand mixer, beat the butter and sugar together unit light and fluffy. Mix in the egg, then add in the flour mixture and mix just until combined.

Press the mixture evenly into the bottom of the prepared pan and up the sides 1/4-inch. Set the pan aside.

Preheat the oven to 350ºF.

In a stand mixer, mix together the sugar, flour and salt. Add the butter and continue to mix until the mixture is crumbly and pieces of butter are very small. You can also do this by hand with a pastry cutter.

Add the walnuts and stir to combine.

Spread the apple mixture evenly over the crust, leaving a 3/4-inch border around the sides.

Sprinkle the crumb topping evenly over the filling.

Bake in the preheated oven until the top is golden brown, about 35 minutes.

Cool completely, then remove from the pan and cut into bars.