Preheat your oven to 350°F (175°C).

In a cast iron skillet, heat olive oil over medium heat. Sauté onion for 3-4 minutes until translucent, then add garlic and cook for another 1-2 minutes.

Add ground beef, salt, pepper, paprika, thyme, and Worcestershire sauce. Cook for 6-8 minutes until browned.

In a bowl, combine sliced potatoes and melted butter, season with salt and pepper, and toss.

Layer half of the potato mixture in a baking dish, add the beef mixture, then top with remaining potatoes.

In a bowl, whisk heavy cream, milk, flour, ground mustard, salt, and parsley until smooth. Pour over casserole.

Sprinkle cheddar and mozzarella cheese evenly over the top.

Cover with foil and bake for 45 minutes. Remove foil and bake for an additional 15 minutes until golden brown.

Let cool for 10 minutes, garnish with fresh parsley, and serve.