Preheat your oven to 350°F (175°C) and generously grease a 9x13-inch baking dish with olive oil or butter to prevent sticking.

In a large skillet over medium-high heat, brown the ground beef with the chopped onion for about 5-7 minutes, breaking up the meat as it cooks. Once no pink remains, drain the excess fat thoroughly, then stir in the minced garlic, salt, pepper, and paprika. Cook for another minute until fragrant.

In a medium bowl, whisk together the cream of mushroom soup and milk until completely smooth with no lumps. This creates your creamy sauce that binds everything together.

Layer half of the sliced potatoes evenly across the bottom of your prepared baking dish, overlapping them slightly for better coverage. Spread half of the beef mixture over the potatoes, pour half of the soup mixture on top, then sprinkle with 1 cup of cheddar cheese.

Repeat the layering process with the remaining potatoes, beef mixture, and soup. Top with the remaining cheddar cheese and mozzarella for that extra gooey finish.

Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and bake for an additional 15-20 minutes until the potatoes are fork-tender and the cheese is golden and bubbly. Let the casserole rest for 5-10 minutes before serving to help the layers set properly.