Preheat oven to 375. Line a 17 x 15 inch rimmed baking sheet or jelly roll pan with parchment paper. In a standing mixer fitted with paddle, beat butter with the sugar at medium speed until light and fluffy, about 2 minutes. At low speed, beat in flour and salt until a soft dough forms. Press dough over bottom of prepared pan and 1/2 inch up the sides in an even layer. Bake in center of oven for about 20 minutes until crust is golden and set. Let cool on rack. Meanwhile, make the filling. In each of 2 large skillets, melt 3 T. of the butter with 1/4 cup of the light brown sugar. Add apples to skillets land cook over high heat, stirring occasionally until softened, about 10 minutes. Stir half of the cinnamon and nutmeg into each skillet. Cook until apples are caramelized and very tender and liquid is evaporated, about 10 minutes longer. Scrap up any bits stuck to the bottom of the skillets; add up to 1/2 cup of water to each pan to prevent scorching. Let cool. Make the topping. Spread walnuts in pie plate; toast until golden, about 8 minutes. Let cool, then coarsely chop. In a bowl, mix oats, flour, light brown sugar, cinnamon, baking soda and salt. Using a pastry blender or 2 knives, cut in butter until mixture resembles coarse meal. Stir in walnuts; press mixture into clumps. Spread apple filling over crust. Scatter crumbs on top, pressing lightly into an even layer. Bake in center of oven for 1 hour, until topping is golden; rotate pan halfway through baking. Let cool completely on rack before cutting into 2 inch bars. To make ahead, the bars can be stored in airtight container at room temperature for 4 days or frozen for up to 1 month. Makes 4 dozen two inch bars.
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