Preheat your oven to 375 degrees Fahrenheit. Grease a large baking dish (about 9x13 inches) with butter or non-stick spray.

In a skillet over medium heat, cook the ground beef, onions, and garlic until the beef is browned and onions are softened. Drain any excess fat.

Arrange a layer of thinly sliced russet potatoes at the bottom of the prepared dish. Season with half of the salt, pepper, and paprika.

Spread the cooked beef and onion mixture evenly over the potatoes.

In a small bowl, whisk together the cream of mushroom soup, milk, and the remaining salt, pepper, and paprika. Pour this mixture over the beef layer.

Cover the dish with foil and bake for 30-40 minutes, or until the potatoes are fork-tender.

Remove the foil, sprinkle the shredded cheddar cheese on top, and optionally add french fried onions. Bake uncovered for an additional 10-15 minutes until the cheese is melted and bubbly.

Let it cool for a few minutes, then garnish with green onions if desired. Serve warm.