Apple Pie Bars

Name: Apple Pie Bars
Ingredients: nonstick cooking spray; 1 cup unsalted butter (room temperature); 1/3 cup granulated sugar; 1 1/2 cups all-purpose flour; 1/4 teaspoon kosher salt; 3 tablespoons unsalted butter; 1/4 cup firmly packed golden brown sugar; 4 large granny smith apples (about 1 lb peeled, cored, and thinly sliced); 2 teaspoons ground cinnamon; 1/4 teaspoon freshly grated nutmeg; 1 cup quick cooking rolled oats; 1/2 cup all-purpose flour (plus 2 tablespoons); 1/2 cup firmly packed golden brown sugar; 1/2 teaspoon cinnamon; 1/4 teaspoon baking soda; 1/4 teaspoon kosher salt; 1/2 cup cold unsalted butter (cut into small cubes); 1/4 cup walnuts toasted (I skip this)