Preheat & Prepare – Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.
Make the Shortbread Crust – In a bowl, mix melted butter, brown sugar, and flour until combined. Press the mixture evenly into the bottom of the prepared pan. Bake for 15 minutes until lightly golden. Set aside.
Prepare the Apple Filling – In a large bowl, toss the sliced apples with sugar, cinnamon, nutmeg, cornstarch, and lemon juice. Spread the apple mixture evenly over the baked crust.
Make the Crumble Topping – In a separate bowl, mix flour, oats, brown sugar, and cinnamon. Cut in the butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Sprinkle evenly over the apples.
Bake the Bars – Return to the oven and bake for 30–35 minutes, or until the topping is golden and the apples are tender.