Place half the sliced potatoes in an even layer in the bottom of the slow cooker.
Add half the onion slices, half the diced ham, and half the shredded cheddar cheese over the potatoes. Sprinkle with half the garlic powder plus salt and black pepper.
Repeat the layers with the remaining potatoes, onion, ham, and cheese. Finish with the rest of the garlic powder, salt, and black pepper.
Pour the heavy cream and milk evenly over the top layers. Dot the surface with the butter.
Cover and cook on low for 6 to 7 hours until the potatoes are tender when pierced with a fork.