Thoroughly coat your 6-quart slow cooker with non-stick cooking spray, focusing on the sides where cake tends to stick most.
Pour the peaches WITH their syrup into the bottom of your prepared slow cooker, spreading them into an even layer.
Sprinkle the cinnamon, nutmeg, and brown sugar evenly over the peaches.
Evenly distribute the dry cake mix over the spiced peaches, creating a complete blanket of mix. Do not stir.
Arrange the thin butter slices across the cake mix surface, trying to cover as much area as possible. Sprinkle nuts over the top if using.
Cover and cook on HIGH for 2.5 to 3 hours or on LOW for 4 to 5 hours. You'll know it's ready when the edges are bubbling and the top develops a golden-brown crust.