Step 1: Preheat your oven to 375°F (190°C). Grease a 9×13-inch casserole dish with nonstick spray or a light coating of olive oil.

Step 2: Prepare the potatoes. Peel and thinly slice the russet potatoes into uniform rounds about 1/8 inch thick. Set them aside in a bowl of cold water to prevent browning.

Step 3: Heat olive oil in a large skillet over medium-high heat. Add the finely chopped onion and cook for 3-4 minutes until translucent. Stir in the minced garlic and cook for an additional 1 minute until fragrant.

Step 4: Add the ground beef to the skillet. Cook, breaking up the meat with a wooden spoon, until it’s fully browned and no pink remains, about 6-7 minutes. Drain any excess fat from the pan. Season the beef mixture with salt, black pepper, and paprika. Remove from heat and set aside.

Step 5: In a medium saucepan, melt the butter over medium heat. Whisk in the flour to create a roux, cooking for 1-2 minutes until it turns a light golden color. Gradually add the milk, whisking constantly to avoid lumps. Continue to whisk until the mixture thickens, about 4-5 minutes. Stir in 2 cups of shredded cheddar cheese and the beef broth. Mix until the cheese is fully melted and the sauce is smooth.

Step 6: Drain and pat the potato slices dry with a clean kitchen towel. Begin layering the casserole: place a single layer of potato slices in the bottom of the prepared casserole dish. Spread a portion of the cooked beef mixture over the potatoes, followed by a generous ladle of the cheese sauce. Repeat these layers until all ingredients are used, finishing with a final layer of cheese sauce.

Step 7: Sprinkle the remaining 1 cup of shredded cheddar cheese over the top of the casserole. Cover the dish with aluminum foil and bake for 45 minutes. Then, remove the foil and bake for an additional 15-20 minutes, or until the potatoes are tender and the cheese is golden and bubbly.

Step 8: Remove the casserole from the oven and let it cool for 10 minutes. Dollop sour cream over the top and sprinkle with sliced scallions. Garnish with chopped fresh parsley if desired.