01 - Combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and salt in a large bowl. Toss until apples are thoroughly coated with the spice mixture.
02 - Transfer the coated apple mixture to a lightly greased 4-quart or larger crockpot, 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 rub into dry ingredients using fingertips or a pastry cutter until mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - Sprinkle the oat crumble mixture evenly over the apples, covering completely.
05 - Cover and cook on HIGH heat for 2 to 2 1/2 hours, or on LOW heat for 3 1/2 to 4 hours, until apples are tender when pierced and topping is golden brown.
06 - For a crunchier texture, remove lid during the final 30 minutes of cooking time to allow steam to escape and topping to crisp further.
07 - Serve warm directly from the slow cooker, optionally topped with vanilla ice cream or freshly whipped cream.