01 - Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a large bowl, combine the sliced apples with apple cider, granulated sugar, lemon juice, cinnamon, nutmeg, allspice, cornstarch, and salt. Toss to coat evenly and set aside.
03 - In a separate bowl, thoroughly combine the flour, rolled oats, light brown sugar, melted butter, baking powder, and kosher salt until a crumbly, moistened mixture forms.
04 - Press two-thirds of the crumble mixture firmly into the bottom of the prepared baking pan to create an even base layer.
05 - Evenly distribute the prepared apple mixture over the crumble base, ensuring all juices are poured over the top.
06 - Sprinkle the remaining crumble mixture evenly over the apple layer.
07 - In another bowl, combine the chopped nuts, crushed pretzels or graham crackers, melted butter, and demerara sugar. Scatter this mixture uniformly over the top layer.
08 - Bake for 40–45 minutes, or until the topping achieves a golden hue and the apple filling is bubbling.
09 - Allow the slab to cool in the pan for at least 1 hour before carefully lifting it out and slicing into portions.