Preheat the oven to 350°F.
In a large mixing bowl, combine the ground beef, breadcrumbs, egg, garlic powder, onion powder, salt, and black pepper. Mix until just combined.
Press half of the meat mixture into the bottom of a greased 9x13-inch baking dish to form a base layer.
Spread the mashed potatoes evenly over the meat layer.
In a separate bowl, mix together the ketchup and barbecue sauce, then spread half of this mixture over the mashed potatoes.
Layer the remaining meat mixture on top of the sauce, pressing it down gently.
Spread the remaining sauce mixture over the meat layer.
Sprinkle the shredded cheddar cheese and cooked bacon bits evenly over the top.
Cover the dish with aluminum foil and bake for 45 minutes.
Remove the foil and bake for an additional 15 minutes, or until the meat is cooked through and the cheese is bubbly.
Let the casserole cool for 5-10 minutes before serving. Drizzle sour cream on top and garnish with chopped green onions.