Begin by washing the three large russet potatoes under cold water to remove any dirt. Next, peel the potatoes using a vegetable peeler and cut them into uniform chunks, about 1-2 inches.

Place the potato chunks in a large pot and cover them with water. Add a generous teaspoon of salt to season the potatoes. Bring the water to a boil over high heat and cook the potatoes for about 15-20 minutes until they are fork-tender.

Once the potatoes are tender, drain them well in a colander and return them to the pot. Using a potato masher, mash the potatoes until they reach your desired consistency.

In a mixing bowl, combine one cup of sour cream, the finely chopped onion, melted butter, salt, and black pepper. Mix thoroughly until well combined.

Stir in the two cups of grated sharp Cheddar cheese into the potato mixture.

Preheat your oven to 350°F (175°C). Transfer the cheesy potato mixture into a greased baking dish, smoothing the top with a spatula.

After baking, carefully remove the dish from the oven and let it cool slightly. Just before serving, sprinkle freshly chopped chives or green onions on top.