Preheat oven to 350°F (175°C).
In a large skillet over medium heat, cook the ground beef and chopped onion until beef is browned and fully cooked. Drain excess fat.
Stir in the salt, pepper, and Worcestershire sauce. Set aside.
In a separate bowl, whisk together the cream of mushroom soup and milk until smooth.
In a greased 9x13-inch baking dish, spread the mashed potatoes evenly on the bottom.
Top with the cooked beef mixture.
Pour the soup mixture evenly over the beef layer.
Sprinkle cheddar cheese on top if using.
Bake for 25–30 minutes, or until bubbly and heated through.
Garnish with chopped parsley if desired. Serve hot.