Place sliced apples in the slow cooker. Add sugars, cinnamon, nutmeg & salt; stir. Let sit while preparing crisp topping.

Combine oats, flour, brown sugar, cinnamon and salt in a large mixing bowl. Stir until well combined using fingertips; work the butter into the oat mixture until the mixture starts to clump together.

Stir in the apple mixture one more time and then spread apples out into an even layer. Sprinkle on the crisp topping.

Cook on high for 2 hours or on low for 3-3 1/2 hours, until apples are soft. Turn off heat and let stand for at least 30 minutes, up to an hour before serving (keeping lid on). Serve with vanilla ice cream, if desired.