Place the sliced apples, granulated sugar, cinnamon, and nutmeg in a large mixing bowl. Toss until evenly coated, then move to the slow cooker and arrange in a single, uniform layer.

In another large mixing bowl, whisk together the brown sugar, old-fashioned oats, and all-purpose flour. Add the butter cubes and work them into the dry mixture with two forks or a pastry cutter until the texture becomes crumbly.

Distribute the oat mixture in an even layer over the apples in the slow cooker.

Put the lid on and cook on high for 2 hours. After that, set to warm, take off the lid, and allow to cool for 30 minutes.

Serve warm and enjoy!