Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 2 1/2-quart casserole dish.

Fill a large pot with lightly salted water and bring to a boil. Add whole unpeeled potatoes and cook until tender, 25 to 30 minutes. Transfer potatoes to a cutting board; discard water. When cool enough to handle, peel potatoes and grate into a medium bowl.

Melt 1/4 cup butter in a large skillet over medium heat. Stir in soup; cook until begins to bubble. Remove from stove; stir in sour cream, Cheddar cheese, and green onion, then stir in potatoes. Pour mixture into the prepared casserole dish.

Shake cornflake crumbs and 2 tablespoons melted butter together in a resealable plastic bag until combined. Sprinkle over top of casserole.

Bake in the preheated oven for 45 minutes.