Preheat your oven to 375°F (190°C).
Wash and peel the potatoes. Cut them into 1/2-inch cubes and set aside.
In a large pot, bring water to a boil and add the cubed potatoes. Cook for about 10 minutes, or until they are just fork-tender. Drain and let them cool slightly.
In a mixing bowl, combine the melted butter, sour cream, milk, garlic powder, onion powder, salt, and black pepper. Whisk until well blended.
Add the cooked potatoes to the mixing bowl and gently fold the mixture until the potatoes are evenly coated.
Transfer the potato mixture to the baking dish, spreading it out evenly.
Sprinkle the shredded cheddar cheese evenly over the top.
Cover the baking dish with aluminum foil and bake for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for a few minutes before serving. Garnish with chopped green onions if desired.