In a medium bowl, whisk together the flour, brown sugar, sugar, cinnamon, nutmeg, and salt. Grate the cold butter into the bowl. Cut in the butter using a pastry blender or two knives, until the pieces are no larger than pea sized. Add the walnuts and toss to combine.
Spray the crock pot with cooking spray, then add the apple slices to the crock pot. Add the sugar, corn starch, cinnamon, and lemon juice, and toss to combine.
Sprinkle the topping in an even layer over the apples. Cover and cook on high for 2 hours, or on low for 4 hours. Partially uncover, and continue cooking for one hour to remove the excess moisture from the topping. Serve warm with ice cream or whipped cream on top.