Preheat the oven to 375°F (190°C) and grease a 9x13-inch casserole dish.
In a large bowl, combine the diced potatoes, shredded chicken, chopped bacon, and ranch dressing.
Incorporate half of the cheddar cheese, garlic powder, and black pepper into the mixture, stirring until well combined.
Transfer the mixture to the prepared casserole dish, spreading it evenly.
Cover the dish with foil and bake for 30 minutes.
Remove the foil, sprinkle with the remaining cheddar cheese, and return to the oven, uncovered, for an additional 15 minutes or until golden and bubbly.
Let the casserole cool for about 10 minutes before serving, garnishing with sliced green onions.