Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish and set it aside.

Place the peeled and cubed russet potatoes in a large pot and cover them with cold water. Add a pinch of salt if you like. Bring the pot to a boil over medium-high heat.

Reduce the heat to a gentle boil and cook the potatoes for about 12–15 minutes, or until they are tender when pierced with a fork.

While the potatoes cook, heat the olive oil in a skillet over medium heat. Add the finely diced yellow onion and cook for 3–4 minutes, stirring, until it softens. Add the minced garlic and cook for another 30 seconds. Remove the skillet from the heat and let the mixture cool slightly.

In a large mixing bowl, combine the ground beef, plain breadcrumbs, eggs, 1/2 cup milk, ketchup, Worcestershire sauce, salt, black pepper, dried Italian seasoning, and the cooked onion and garlic mixture.

Use clean hands or a sturdy spoon to mix everything together just until combined. Do not overmix, or the meatloaf layer can turn dense.

Transfer the meat mixture to the prepared baking dish. Press it into an even layer across the bottom of the dish, forming a flat meatloaf base.

Place the baking dish with the meatloaf base into the preheated oven and bake for 20 minutes while you finish the potatoes.

Once the potatoes are tender, drain them well and return them to the warm pot. Add the 3 tablespoons unsalted butter, 1/2 cup milk (for potatoes), 1 teaspoon salt (for potatoes), and 1/2 teaspoon black pepper (for potatoes).

Mash the potatoes until they are smooth and creamy, or leave them a bit chunky if you prefer a rustic texture. Taste and adjust the seasoning if needed.

Carefully remove the partially baked meatloaf base from the oven. Spoon the mashed potatoes over the meat layer and spread them gently into an even layer, covering the meatloaf completely.

Sprinkle the shredded cheddar cheese evenly over the mashed potato layer, making sure it reaches the edges for a full cheesy topping.

Return the baking dish to the oven and bake for another 20–25 minutes, or until the cheese is melted and bubbly and the meat is cooked through.

If you like a slightly more golden top, you can place the casserole under the broiler for 1–2 minutes, watching closely so the cheese does not burn.

Remove the Cheesy Meatloaf Casserole from the oven and let it rest for about 10 minutes so the layers can set slightly.

Sprinkle the chopped fresh parsley over the top for a bit of color, if using. Slice the casserole into portions and serve warm.