Spray the inside of the crockpot with cooking spray. This helps prevent sticking and makes cleaning much easier. Set the slow cooker aside while you prepare the remaining ingredients.

Wash, peel, and thinly slice the potatoes. Try to keep every slice about the same thickness. Even slices cook evenly and create a creamy texture throughout the dish.

In a mixing bowl, whisk together the milk, heavy cream, flour, melted butter, garlic, salt, pepper, onion powder and parsley. Mix until smooth with no flour lumps remaining.

Place one layer of potatoes into the crockpot. Add some onions, diced ham, shredded cheese and a little cream sauce. Repeat these layers until every ingredient has been used.

Pour the remaining cream sauce over the top. Sprinkle the last handful of cheese evenly across the potatoes. This creates a rich and cheesy top layer.

Cover the crockpot with the lid. Cook on low for about 6 hours or on high for about 3½ hours. The potatoes should become soft when pierced with a fork.

Turn off the slow cooker and let the dish rest for about 10 minutes. This allows the creamy sauce to thicken before serving. Garnish with fresh parsley if desired.