Begin by washing and peeling the russet potatoes under cool running water. Cut them into even chunks, about 1-2 inches in size.

Place the chopped potatoes into a large pot and cover them with cold salted water. Bring to a boil, then reduce to a simmer. Cook for 15-20 minutes until fork-tender.

Carefully drain the potatoes and return them to the pot over low heat. Mash the potatoes until you reach a smooth and creamy consistency with a few lumps for texture.

In a separate bowl, whisk together the sour cream, finely chopped onion, melted butter, salt, and black pepper. Fold this mixture into the mashed potatoes.

Stir in the grated cheddar cheese until fully incorporated. Taste and adjust seasoning as needed.

Transfer the potato mixture to a greased baking dish, smoothing the top. Dot with remaining melted butter. Bake at 350°F for 25-30 minutes until bubbly and golden.

Let cool slightly before serving, then garnish with finely chopped chives or green onions.