Preheat the oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.

Boil potatoes in a large pot of salted water until fork-tender, about 15–20 minutes.

While potatoes cook, heat a skillet over medium heat. Add ground beef, diced onion, and minced garlic. Cook until beef is browned and onions are translucent. Drain excess fat.

Stir Worcestershire sauce into the beef mixture and season with salt and pepper to taste.

Drain potatoes and return to pot. Mash with butter and milk until smooth. Season with salt and pepper.

Spread the beef mixture in the prepared baking dish. Top evenly with mashed potatoes and sprinkle half of the shredded Cheddar cheese on top.

Bake for 25–30 minutes, or until the casserole is heated through and the cheese is bubbly and lightly golden.

Remove from oven, sprinkle with remaining Cheddar cheese and chopped parsley. Let stand 5 minutes before serving.