01 - Preheat oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - In a large bowl, combine all-purpose flour, granulated sugar, and salt. Cut in cold diced butter with a pastry cutter or fork until the mixture resembles coarse crumbs.
03 - Firmly press two-thirds of the base mixture into the bottom of the prepared pan, creating an even layer.
04 - In a separate bowl, toss diced apples with lemon juice, granulated sugar, ground cinnamon, and cornstarch until evenly coated.
05 - Spread the apple filling evenly over the base layer in the baking pan.
06 - In another bowl, mix rolled oats, flour, light brown sugar, ground cinnamon, and salt. Stir in melted butter until the mixture forms moist clumps.
07 - Sprinkle the crumble mixture evenly over the apple layer. Bake for 40–45 minutes, or until golden and bubbling at the edges.
08 - Allow bars to cool completely in the pan on a rack before lifting out and slicing.