In a skillet over moderate heat, brown the ground beef along with the finely chopped onion until the meat is no longer pink and is cooked through. Drain any rendered fat and set the mixture aside.
In a separate bowl, whisk together the cream of chicken soup, cream of mushroom soup, milk, garlic powder, paprika, and a pinch each of salt and pepper until the mixture is smooth and combined.
Lightly grease the inside of the slow cooker using nonstick spray or a small amount of butter to prevent sticking. Arrange half of the potato slices on the bottom of the cooker. Spoon over half of the cooked beef and onion mixture. Pour half of the soup mixture evenly over the beef layer. Scatter 1 cup of shredded cheddar across the top.
Repeat the layering process with the remaining potato slices, beef mixture, soup blend, and the final cup of shredded cheese.
Secure the lid and cook on the LOW setting for 6 to 7 hours, or on HIGH for 3 to 4 hours, until the potatoes are fork-tender.
Once done, garnish the top with a light sprinkle of dried parsley and serve the casserole warm.