Step 1: Preheat the Oven
Set your oven to 375°F (190°C) and lightly grease a 9×13-inch baking dish.

Step 2: Make the Meatloaf Mixture
In a large mixing bowl, combine the ground beef, chopped onion, minced garlic, breadcrumbs, milk, egg, Worcestershire sauce, salt, and pepper. Mix gently until all ingredients are fully combined, but don’t overmix as that can make the meatloaf dense.

Step 3: Shape and Bake the Meatloaf Base
Transfer the meat mixture to the greased baking dish and press it into an even layer. Spread the 1/2 cup of ketchup over the top of the meat to create a flavorful glaze. Bake in the preheated oven for 30 minutes.

Step 4: Boil the Potatoes
While the meatloaf layer is baking, place the peeled and sliced potatoes into a large pot of salted water. Bring to a boil and cook for about 15 minutes, or until the potatoes are fork-tender.

Step 5: Mash the Potatoes
Drain the cooked potatoes and return them to the pot. Add the sour cream, milk, butter, and season with salt and pepper to taste. Mash until smooth and creamy. You can use a hand masher or hand mixer depending on your preferred texture.

Step 6: Layer the Casserole
After the meatloaf has baked for 30 minutes, remove it from the oven. Carefully spread the mashed potato layer over the meatloaf using a spatula. Smooth the top evenly.

Step 7: Add Cheese and Bake Again
Sprinkle shredded cheddar cheese over the mashed potatoes. Return the baking dish to the oven and bake for another 15–20 minutes, or until the cheese is melted and bubbly.

Step 8: Garnish and Serve
Remove from the oven and let the casserole rest for 5 minutes. Top with chopped green onions for a pop of color and fresh flavor. Slice and serve warm.