Line a 9x13 inch pan with parchment paper. (This is optional. It makes it really easy to lift the bars out of the pan and slice them.) 

In a large bowl or stand mixer, beat 1 pound of butter until smooth and creamy. Add 3/4 cup sugar, 1/2 cup packed brown sugar, and 2 teaspoons vanilla and beat for 1-2 minutes, remembering to scrape the bottom and sides of the bowl. It should be light and creamy. 

Add 4 cups of flour and 1 and 1/2 teaspoons salt. Beat until just combined. Do not over mix. 

Add about 2/3 of the dough to the prepared pan. (Leave the remaining 1/3 of the dough in the bowl; we're not done with it yet)

Flour your hands lightly and press the dough into the bottom and up the sides of the pan. The crust should go about a half inch up the sides of the pan. Use a glass or a measuring cup to press the edges and bottom. 

Refrigerate the crust for 20 minutes. Don't skip this step! Cold shortbread going into a hot oven is what makes for amazing, tender crusts.

Preheat your oven to 375 degrees F. 

Bake the crust for about 18 minutes, until the the crust is golden brown. Set aside. Lower the oven temperature to 350 degrees F. 

Meanwhile, add the walnuts (or pecans) to the remaining dough in the mixing bowl. Add 1 teaspoon cinnamon and 1 teaspoon nutmeg. (The spices are totally up to you; add as little or as much as you like)

Mix on low speed just until combined. Cover the bowl and stick it in the fridge. 

For the apple filling, start by prepping your apples. You can use any combination of apple flavors that you like. A variety is always best for baking. (Granny Smith, Fuji, Golden Delicious, Pink Lady are all good options. I used half Golden Delicious and half Granny Smith.)

Squeeze about 2 tablespoons fresh lemon juice into a large bowl.

Peel the apples, core, and cut into 1/8 inch slices. See photos for my method. The thinner you slice them, the better!

As you slice the apples, toss them into the bowl with the lemon juice and mix around to coat them. Stir every couple minutes as you are slicing. This will stop your apples from getting brown.

In a large 10 or 12 inch skillet, melt 4 tablespoons butter over medium heat. Add the sliced apples (and any juice in the bowl), 1/4 cup sugar, 1 teaspoon cinnamon, and 1/2 teaspoon nutmeg. (Again, the spice level is totally up to you. Some people don't love nutmeg and you can just omit it entirely if you want.)

Simmer the apples over medium heat, stirring often, for about 12-15 minutes. If it looks like they are boiling too hard, lower the heat to medium-low. When the apples are tender and the liquid has mostly evaporated, turn off the heat. 

Pour the apples into the baked crust, leaving a half inch border along the edge.

Get the remaining crust dough out of the fridge. Use your fingers to pinch medium pieces of dough and drop them over the apples (see photos). 

Bake at 350 fro 25-30 minutes, until the top is golden brown. Cool and cut into bars. OR don't cool, cut out a falling-apart-piece right away, and top it with vanilla ice cream and My Favorite Caramel Sauce. You really can't go wrong here!