Preheat the oven to 350°F (175°C).

Grease a 9x13-inch baking dish with cooking spray or butter.

In a large skillet over medium heat, cook the ground beef and diced onion until the beef is browned and the onion is softened, about 8 to 10 minutes.

Drain excess grease if necessary.

Stir in the minced garlic, Italian seasoning, paprika, salt, and black pepper. Cook for 1 minute until fragrant.

In a medium bowl, whisk together the condensed cream of mushroom soup, milk, sour cream, and melted butter until smooth.

Arrange half of the sliced potatoes in an even layer in the prepared baking dish.

Spread half of the cooked beef mixture over the potatoes.

Pour about half of the soup mixture evenly over the beef layer.

Sprinkle with 1 cup of shredded cheddar cheese.

Repeat the layers with the remaining potatoes, beef mixture, soup mixture, and remaining cheese.

Cover the casserole dish tightly with aluminum foil.

Bake for 60 minutes, or until the casserole is bubbling around the edges.

Remove the foil and continue baking for 20 more minutes, or until the potatoes are tender and the cheese is bubbly and golden.

Let the casserole rest for 5 to 10 minutes before serving.

Garnish with chopped parsley if desired and serve warm.