Preheat the oven to 375°F. Line the bottom and sides of a 13×9-inch baking pan with parchment paper.

Place the softened butter, granulated sugar, brown sugar and vanilla into the bowl of a stand mixer with a paddle attachment. Blend the ingredients on medium speed for two minutes until the mixture is light and creamy.

Sift the flour and salt together in a separate bowl; run the mixer on medium-low speed as you slowly add in the dry ingredients. Scrape the bowl once or twice and mix until everything is just combined.

Set aside one-third of the dough. Drop the rest of the dough in clumps over the bottom of the prepared pan. Press the dough down with your fingers to make an even layer across the bottom of the pan and a half-inch up the sides. Refrigerate the dough in the pan for 20 minutes. Then, bake for 18-20 minutes or until the crust is golden brown. Move it to a rack to cool.

Lower the oven temperature to 350°. Place the dough that you set aside into a mixing bowl; add in the chopped walnuts and teaspoon of cinnamon. Pulse the mixer until everything is combined. Hold this topping aside.

Add all of the sliced apples and lemon juice to a very large bowl, and stir them together to coat the apples in the juice. Add in the ¼ cup of granulated sugar, a teaspoon of cinnamon and ground nutmeg, and mix everything together.

Melt the four tablespoons of butter in a large pot over medium heat. Add in the apple mixture, and bring it to a simmer. Cook the mixture, stirring often, for 12-15 minutes until most of the liquid has evaporated, and the apples are very soft.

Spoon the apple filling over the baked crust, making an even layer and leaving a ½-inch border all around. Take the dough topping and drop pinches of it all over the top of the apples.

Slide the pan into the oven and bake it for 25-30 minutes, or until the topping is browned. Move the pan to a cooling rack and allow it to cool completely. Once that’s done, use the edges of the parchment paper to lift the whole thing out and onto a cutting board. Pull the parchment out from underneath. Slice the bake into 12 bars and serve.