01 - Combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and salt in a large bowl. Toss thoroughly until all apples are evenly coated with the sugar-spice mixture.
02 - Transfer the coated apple mixture to a lightly greased 4-quart or larger slow cooker, spreading them 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 the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs with pea-sized butter pieces remaining.
04 - Sprinkle the crumble mixture evenly over the apples, ensuring complete coverage.
05 - Cover and cook on HIGH setting for 2 to 2.5 hours, or on LOW setting for 3.5 to 4 hours, until apples are tender when pierced and the topping is golden brown and set.
06 - For a crisper texture, remove the lid during the final 30 minutes of cooking time to allow steam to escape and the topping to firm up.
07 - Serve the apple crisp warm, optionally topped with vanilla ice cream or whipped cream for a classic finishing touch.