01 - In a large bowl, combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and a pinch of salt. Toss until apples are well coated with the spice mixture.
02 - Transfer the apple mixture to the bottom of a lightly greased crockpot (at least 4-quart capacity), spreading evenly to create a uniform layer.
03 - In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add in the cold, cubed butter. Using your fingertips or a pastry cutter, rub the butter into the dry ingredients until the mixture resembles coarse crumbs.
04 - Sprinkle the topping evenly over the apples in the crockpot, ensuring complete coverage of the fruit layer.
05 - Cover and cook on HIGH for 2 to 2 1/2 hours, or on LOW for 3 1/2 to 4 hours, until the apples are tender and the topping is golden and crisp.
06 - If desired, remove the crockpot lid and cook uncovered for the last 30 minutes to allow additional crisping of the topping.
07 - Serve warm, optionally with vanilla ice cream or whipped cream for added richness.