In a large bowl, toss together apples, grated apple, sugar, brown sugar, cinnamon, nutmeg and lemon juice. Combine thoroughly and pour the mixture into the bottom of a slow cooker and spread evenly.
In a medium bowl, whisk together oats, flour, sugar, cinnamon and salt. Using your fingers or a pastry cutter, work the butter into the oat mixture until coarse crumbs form.
Sprinkle the oat mixture evenly over the apples.
Place the lid on the slow cooker and cook for 4 hours on high or 7 hours on low until the crust is a golden brown.
Serve hot or at room temperature with whipped cream or ice cream, if desired.