01 - Combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and salt in a large bowl. Toss thoroughly until apples are evenly coated.
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 rub into dry ingredients using fingertips or a pastry cutter until mixture resembles coarse crumbs.
04 - Sprinkle the crumb topping 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 and topping is golden and crisp.
06 - For a crisper topping, remove lid during the final 30 minutes of cooking time.
07 - Serve warm, optionally topped with vanilla ice cream or whipped cream.