Wash and peel the russet potatoes, then cut them into uniform chunks.

Place the potato chunks in a large pot and cover with cold water.

Add a pinch of salt and bring to a boil over medium-high heat. Cook for 15-20 minutes until fork-tender.

Drain the potatoes and return them to the pot. Mash until creamy, leaving some lumps for character.

In a separate bowl, whisk together sour cream, onion, melted butter, salt, and pepper. Pour over the mashed potatoes and fold gently.

Stir in grated cheddar cheese until fully incorporated.

Preheat oven to 350°F (175°C). Transfer mixture to a greased baking dish and dot with remaining butter. Bake for 25-30 minutes until golden brown and bubbly.

Garnish with freshly chopped chives or green onions just before serving.