Slice the potatoes fairly thin. I left the skins on but you can peel them if you like. Once sliced, place the potatoes in cold water so they do not turn brown.
Slice then roughly chop the onion. Add oil to a hot pan, cook the onions until caramelize, then set them aside.
Add the butter to the pan then stir in the minced garlic, then add the flour. Stir until the flour has cooked down. Stir in the cream, milk, cream cheese, 1/2 cup cheddar cheese, and 1/2 cup mozzarella cheese. Stir until everything is melted and blended together. Taste for salt, and season to your liking.
Drain, dry, then toss in the sliced potatoes. Stir until all of the potato slices are covered in the cheese sauce.
Layer half of the potatoes into your slow cooker, ten add half of the caramelized onions, half of the remaining cheddar cheese, half of the remaining mozzarella cheese. Repeat using the other half of the ingredients.
Pour any remaining cheese sauce over the top. Cover and cook on the high setting for 3 hours.