01 - In a large bowl, combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and salt. Toss until apples are thoroughly coated with the sugar mixture.
02 - Transfer the apple mixture to the bottom of a lightly greased 4-quart or larger crockpot, spreading into an even layer.
03 - In a separate bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add cold cubed butter and rub into the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs.
04 - Sprinkle the crumb topping evenly over the apples. 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.
05 - For a crisper topping, remove the lid during the final 30 minutes of cooking time.
06 - Serve warm, optionally topped with vanilla ice cream or whipped cream.