Bring a large pot of water to a boil, then add the whole unpeeled russet potatoes and boil for 20-25 minutes until just tender. Visual cue: a knife should slide in with only slight resistance.

Drain the potatoes and cool completely, then peel and shred them. Visual cue: shreds should look fluffy and loose rather than mashed.

Preheat the oven to 350°F, then butter a 9x13-inch baking dish. Visual cue: the dish should be lightly coated with melted butter.

In a mixing bowl, combine sour cream, 1 cup shredded sharp cheddar, grated Parmesan, sliced green onions, melted butter, salt, black pepper, and garlic powder. Visual cue: the mixture should look smooth and evenly speckled.

Gently fold the shredded potatoes into the sour cream mixture until coated throughout. Visual cue: potato shreds should be glossy and creamy, not dry.

Spread the potato mixture into the buttered baking dish, then sprinkle with the remaining shredded sharp cheddar. Visual cue: an even, thick cheese layer should cover the top.

Bake at 350°F for 40-45 minutes until hot and golden on top. Visual cue: edges should bubble and the surface should be browned and set.

Garnish with fresh chives, then serve. Visual cue: chives should sit fresh and bright on the golden crust.