Preheat your oven to 375°F (190°C) and grease a 9×13 inch baking dish.
In a large bowl, combine the ground beef, onion, breadcrumbs, milk, egg, Worcestershire sauce, salt, pepper, and garlic powder. Mix until just combined; press the meat mixture evenly into the prepared baking dish.
Spread the creamy mashed potatoes evenly over the meat mixture.
Sprinkle shredded cheddar cheese over the mashed potatoes followed by crumbled bacon and chopped green onions.
Bake in your preheated oven for about 35–40 minutes until the cheese is melted and bubbly.
Allow your casserole to cool for a few minutes before slicing and serving.