Preheat oven to 350° F. Place potatoes, celery, and onion in large pot or Dutch oven. Add just enough water to cover vegetables.

Cook, covered, until vegetables are almost tender, 8-10 minutes. Drain vegetables and return to heat. Add all ¼ cups of cheddar cheese, butter, cream cheese, parsley, salt, pepper, and garlic powder. Stir to combine thoroughly.

Transfer to greased 13x9-inch baking dish. Top with green onions and remaining cheddar cheese. Bake, uncovered, 15-20 minutes or until bubbly and cheese is melted. To serve, sprinkle with additional parsley and green onions for garnish.