01 - In a large bowl, combine sliced apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and a pinch of salt. Toss until the apples are evenly coated.
02 - Transfer the apple mixture to the bottom of a lightly greased 4-quart or larger slow cooker, spreading it into an even layer.
03 - In a separate bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add the cold cubed butter and work it into the dry ingredients using your fingertips or a pastry cutter until the mixture forms coarse crumbs.
04 - Sprinkle the crumb mixture evenly over the apples in the slow cooker.
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 - Remove the lid and continue cooking uncovered for the last 30 minutes to allow the topping to set and crisp further.
07 - Serve warm, optionally accompanied by vanilla ice cream or whipped cream.