Step 1: Preheat the Oven and Prepare the Baking Dish

• Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with butter or cooking spray to prevent sticking.

Step 2: Cook the Beef Mixture

• Heat 2 tablespoons of olive oil in a skillet over medium heat.
• Add the diced onion and sauté for 3 minutes until softened.
• Stir in the minced garlic and cook for another 30 seconds.
• Add the ground beef and cook until browned, breaking it up with a spatula.
• Season with salt, black pepper, smoked paprika, dried thyme, and Worcestershire sauce.
• Once cooked, drain any excess fat and set the beef aside.

Step 3: Slice and Season the Potatoes

• Wash and peel the russet potatoes.
• Slice them thinly (about ⅛-inch thick) using a sharp knife or mandoline slicer.
• In a bowl, toss the potato slices with melted butter, salt, garlic powder, and black pepper.

Step 4: Make the Creamy Sauce

• In a saucepan over medium heat, melt 2 tablespoons of butter.
• Whisk in the flour and cook for 1 minute to remove the raw flour taste.
• Gradually whisk in the milk and heavy cream, stirring constantly to avoid lumps.
• Add salt, ground mustard, and dried parsley.
• Continue cooking until the sauce thickens slightly, about 3-5 minutes.
• Remove from heat and stir in 1 cup of shredded cheddar cheese until melted and smooth.

Step 5: Assemble the Casserole

• Spread half of the sliced potatoes in an even layer at the bottom of the baking dish.
• Spoon half of the cooked ground beef over the potatoes.
• Pour half of the creamy cheese sauce evenly over the beef.
• Repeat with the remaining potatoes, beef, and sauce.
• Sprinkle the top with mozzarella and cheddar cheese.

Step 6: Bake the Casserole

• Cover the dish with foil and bake for 40 minutes.
• Remove the foil and bake for another 20 minutes, or until the top is golden brown and bubbly.
• Let the casserole sit for 10 minutes before serving.

Step 7: Garnish and Serve

• Sprinkle chopped fresh parsley on top for color and freshness.
• Serve warm and enjoy!