Grease the slow cooker with butter or non-stick spray.
Add the sliced apples to the slow cooker.
In a bowl, mix brown sugar, flour, oats, cinnamon, and nutmeg.
Cut in the butter until the mixture resembles coarse crumbs.
Sprinkle the mixture evenly over the apples.
Cover and cook on low for 4 hours, or until apples are tender.
Serve warm, optionally with vanilla ice cream.