Step 1: In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 2: Slowly add the heavy cream to the roux while stirring continuously until the sauce thickens and is smooth.
Step 3: Season the creamy sauce with garlic powder, salt, and pepper. Stir well to combine.
Step 4: Lightly grease the inside of your slow cooker. Layer one-third of the thinly sliced potatoes at the bottom of the slow cooker.
Step 5: Top the potatoes with one-third of the chopped onions, followed by one-third of the shredded cheddar cheese and one-third of the shredded mozzarella cheese.
Step 6: Repeat the layering process two more times, ending with a layer of cheese on top.
Step 7: Carefully pour the prepared creamy sauce evenly over all the layers in the slow cooker, ensuring it seeps down.
Step 8: Cover the slow cooker with the lid and cook on LOW for 6-7 hours, or on HIGH for 3-4 hours, or until the potatoes are fork-tender and the sauce is bubbly and hot.
Step 9: Before serving, sprinkle extra cheese (if desired) and fresh chopped parsley on top for garnish.