Prepare the Mashed Potatoes: While the meatloaf is baking, place the potato chunks in a large pot and cover with cold water. Bring to a boil over medium-high heat and cook for 15-20 minutes, or until the potatoes are fork-tender. Drain the potatoes and return them to the pot. Add the milk, butter, sour cream, salt, and black pepper. Mash until smooth and creamy. Adjust seasoning as needed.
Assemble and Bake: Spread the mashed potatoes evenly over the top of the cooked meatloaf. Sprinkle the shredded cheddar cheese over the mashed potatoes. Return the meatloaf to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Serve: Let the meatloaf cool for about 5 minutes before slicing. Garnish with chopped fresh parsley if desired. Serve warm and enjoy!