01 - Combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and salt in a large bowl. Toss until apples are evenly coated with the spice mixture.
02 - Transfer the coated apple mixture to a lightly greased 4-quart or larger slow cooker, spreading into an even layer across the bottom.
03 - Mix oats, flour, brown sugar, cinnamon, and salt in a separate bowl. Add cold cubed butter and work into dry ingredients using fingertips or pastry cutter until mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - Sprinkle the crumble mixture evenly over the apples, covering completely.
05 - Cover and cook on HIGH for 2 to 2.5 hours or on LOW for 3.5 to 4 hours until apples are tender when pierced and topping is golden brown.
06 - Remove lid for the final 30 minutes of cooking to allow the topping to become crisp and lightly browned.
07 - Serve warm, optionally topped with vanilla ice cream or whipped cream.