Begin by peeling and chopping the russet potatoes into even chunks, about 1 to 2 inches in size. Place them in a large pot with enough salted water to cover the potatoes. Bring to a boil over medium-high heat, then reduce to a simmer for 15-20 minutes until fork-tender.

Carefully drain the potatoes in a colander and let them steam dry for a minute. Transfer back into the warm pot and mash until smooth, aiming for a fluffy mash.

While the potatoes are still warm, mix in the sour cream, melted butter, sharp cheddar, salt, and pepper. Stir until well combined and cheese melts smoothly.

Preheat your oven to 350°F (175°C). Grease a baking dish with butter or oil. Spoon the creamy mixture into the dish and smooth the top. Sprinkle additional cheddar over the surface.

Bake for 20-25 minutes until the top is golden and bubbling, then remove from the oven and let cool slightly.

Sprinkle chopped chives or green onions over the top before serving for color and enhanced flavor.