Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the lean ground beef, diced onion, parsley, Italian bread crumbs, milk, Worcestershire sauce, eggs, garlic powder, salt, and black pepper. Mix until just combined.
Transfer the meat mixture into your baking dish and shape it into a loaf.
In a separate bowl, mix together the ketchup, balsamic vinegar, and brown sugar. Spread this glaze over the top of the meatloaf.
Place the baking dish in the preheated oven and bake for 45 minutes.
After 45 minutes, remove the casserole from the oven and spread the mashed potatoes over the top.
Sprinkle shredded cheddar cheese over the mashed potatoes.
Return the casserole to the oven and bake for an additional 15-20 minutes.
Remove the casserole from the oven and let it rest for about 10 minutes before serving.