01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing an overhang for easy removal.
02 - In a medium bowl, combine flour, oats, sugar, and salt. Add cold cubed butter and blend with a pastry cutter or your fingers until the mixture forms coarse crumbs. Press firmly and evenly into the base of the prepared pan.
03 - Bake the crust for 10 minutes. Remove from the oven and set aside while preparing the filling.
04 - In a large bowl, toss diced apples with both sugars, flour, cinnamon, nutmeg, and lemon juice until evenly coated. Layer the apple mixture over the pre-baked crust.
05 - In a medium bowl, combine oats, flour, brown sugar, salt, and cinnamon. Add melted butter and mix until the mixture is crumbly. Evenly sprinkle the topping over the apples.
06 - Bake for 30 to 35 minutes, or until the topping is golden brown and apples are bubbling.
07 - Allow the bars to cool completely in the pan. Use the parchment overhang to lift out, then cut into bars for serving.