Prepare the Meatloaf Base
Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine ground beef, onion, garlic, Worcestershire sauce, breadcrumbs, egg, salt, and pepper. Mix until evenly incorporated.
Cook the Meatloaf Layer
Press the meat mixture into a greased baking dish, forming an even layer. Bake for 20-25 minutes, or until the beef is fully cooked. Remove from the oven and drain any excess grease.
Prepare the Mashed Potatoes
If not using leftovers, prepare mashed potatoes by boiling peeled potatoes until fork-tender. Mash with milk, butter (if desired), and a pinch of salt.
Assemble the Casserole
Spread the mashed potatoes evenly over the cooked meatloaf layer. Sprinkle cheddar and mozzarella cheese on top.
Bake and Add Toppings
Return the casserole to the oven and bake for 10-15 minutes, or until the cheese is melted and bubbly.
For extra crunch, sprinkle crispy fried onions or bacon bits over the cheese before serving.
Garnish and Serve
Garnish with parsley and serve warm.