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 with the spice mixture.
02 - Transfer the seasoned apple mixture to a lightly greased 4-quart or larger slow cooker. Spread the apples in an even layer across the bottom.
03 - Combine oats, flour, brown sugar, cinnamon, and salt in a separate bowl. Add cold cubed butter and work into dry ingredients using fingertips or a pastry cutter until mixture resembles coarse crumbs.
04 - Sprinkle the crumb topping evenly over the apple layer in the slow cooker, ensuring complete coverage.
05 - Cover and cook on HIGH setting for 2 to 2.5 hours, or on LOW for 3.5 to 4 hours, until apples are tender and topping is golden brown.
06 - For a crunchier texture, remove the lid during the final 30 minutes of cooking to allow the topping to crisp further.
07 - Serve warm, optionally topped with vanilla ice cream or whipped cream.