01 - Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a large bowl, combine rolled oats, all-purpose flour, light brown sugar, ground cinnamon, and salt. Stir in the melted unsalted butter until the mixture resembles coarse crumbs.
03 - Reserve approximately 1 cup of the prepared oat mixture for the topping. Firmly press the remaining mixture into the bottom of the prepared baking pan to form the crust.
04 - In a medium bowl, toss the diced apples with granulated sugar, all-purpose flour, ground cinnamon, ground nutmeg, and lemon juice until evenly coated.
05 - Spread the coated apple mixture evenly over the pressed crust.
06 - Evenly sprinkle the reserved oat mixture over the apple layer, pressing down lightly.
07 - Bake for 40 to 45 minutes, or until the topping is golden brown and the apple filling is bubbling. Allow to cool completely in the pan on a wire rack before lifting out by the parchment overhang and cutting into bars.