Ingredients
For the Crust
3 sticks unsalted butter, softened
3/4 cup granulated sugar
3 cups all-purpose flour
1/2 teaspoon kosher salt
For the Filling
6 tablespoons unsalted butter
1/2 cup dark brown sugar
12 fresh, crisp apples (about 6 pounds), peeled, cored and thinly sliced
1 tablespoon ground cinnamon
1/4 teaspoon ground nutmeg
1 teaspoon pure vanilla extract
1 cup water, as necessary
For the Topping
1 cup walnuts, toasted and roughly chopped
3 cups old-fashioned oats
2 cups all-purpose flour
1 1/2 cups dark brown sugar
1 1/4 teaspoons ground cinnamon
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
3 sticks unsalted butter, cubed and chilled
Directions
Preheat the oven to 375F. Line a 12-by-17-inch rimmed baking sheet with silpat or parchment paper.
In a stand electric mixer fitted with the paddle attachment, beat the butter with the sugar at medium speed until light and fluffy, about 2 minutes. At low speed, beat in the flour and salt until a soft dough forms. Press the dough over the bottom of the prepared pan and 1/2 inch up the side in an even layer. Bake in the center of the oven for about 20 minutes, until the crust is golden and set. Let cool on a rack.
In a very large skillet (or do in batches; halving all ingredients), melt butter with the dark brown sugar. Add the apples and cook over high heat, stirring occasionally, until softened, about 15 minutes. Stir in cinnamon, nutmeg and vanilla. Cook until the apples are caramelized and very tender and the liquid is evaporated, about 15 minutes longer; scrape up any bits stuck to the bottom of the skillets and add up to a cup of water to the pan to prevent burning. Let cool.
In a large bowl, mix the oats with the flour, dark brown sugar, cinnamon, baking soda and salt. Using a folk (or your hands), cut in the butter until the mixture resembles large peas. Stir in the walnuts and press the mixture into clumps.
Spread the apple filling over the crust. Scatter the crumbs on top, pressing them gently into an even layer. Bake in the center of the oven for 45 minutes, until the topping is golden. Let cool completely on a rack before cutting into 2-inch bars.
