Crust:
3 sticks unsalted butter, softened
3/4 cup sugar
3 cups all-purpose flour
1/2 teaspoon kosher salt
Filling:
6 tablespoons unsalted butter
1/2 cup light brown sugar
12 Granny Smith apples (about 6 pounds)—peeled, cored and thinly sliced
1 tablespoon cinnamon
1/4 teaspoon freshly grated nutmeg
1 cup water, as necessary
Topping:
3/4 cup walnuts
3 cups quick-cooking oats
2 cups all-purpose flour
1 1/2 cups light brown sugar
1 1/4 teaspoons cinnamon
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
3 sticks unsalted butter, cut into 1/2-inch cubes and chilled
Crust Directions:
- Preheat the oven to 375°
- Line a 15-by-17-inch rimmed baking sheet with parchment paper
- In a standing electric mixer fitted with the paddle, beat the butter with the sugar at medium speed until light and fluffy, 2 minutes. At low speed, beat in the flour and salt until a soft dough forms.
- Press the dough over the bottom of the sheet and 1/2 inch up the side. Bake in the center of the oven for 20 minutes, until the crust is golden. Let cool on a rack.
Filling Directions:
- In each of 2 large skillets, melt 3 tablespoons of the butter with 1/4 cup of the light brown sugar. Add the apples to the skillets and cook over high heat, stirring occasionally, until softened, about 10 minutes.
- Stir half of the cinnamon and nutmeg into each skillet. Cook until the apples are caramelized and very tender and the liquid is evaporated, about 10 minutes longer; scrape up any bits stuck to the bottom of the skillets and add up to 1/2 cup of water to each pan to prevent scorching.
- Let cool.
Topping Directions:
- Spread the walnuts in a pie plate and toast until golden and fragrant, about 8 minutes.
- Let cool, then coarsely chop the walnuts. In a large bowl, mix the oats with the flour, light brown sugar, cinnamon, baking soda and salt.
- Using a pastry blender or two knives, cut in the butter until the mixture resembles coarse meal.
- Stir in the walnuts and press the mixture into clumps.
- Spread the apple filling over the crust.
- Scatter the crumbs on top, pressing them lightly into an even layer.
- Bake in the center of the oven for about 1 hour, until the topping is golden; rotate the pan halfway through baking.
- Let cool completely on a rack before cutting into 2-inch bars.
The bars can be stored in an airtight container at room temperature for 4 days or frozen for up to a month