Preheat the oven to 375°F (190°C). Grease a 9x13 baking dish.
Make the meat mixture: In a large bowl, mix ground beef, egg, breadcrumbs, diced onion, Worcestershire sauce, garlic powder, salt, and pepper.
Form the base layer: Press the meat mixture evenly into the baking dish.
Add mashed potatoes: Spoon mashed potatoes over the beef layer and spread evenly.
Top with cheese: Sprinkle shredded cheddar cheese generously across the top.
Bake: Bake uncovered for 30–35 minutes or until meat is cooked through and cheese is bubbly and golden.
Rest: Let casserole rest for 5 minutes before serving.
Serve and enjoy: Garnish with optional bacon bits or green onions if desired.