1/2 cup (1 stick) unsalted butter, plus more for greasing
1/2 cup milk
3 green onions, chopped
3/4 cup grated Cheddar
Salt and freshly ground black pepper
Preheat the oven to 400 degrees F.
Bake the potatoes until tender, 1 hour. Let cool enough to handle, and then scoop out the flesh into a medium bowl. Warm the butter and milk together in the microwave until the butter is just melted and the milk is warm. Add half the butter and milk to the potatoes and mash well with a potato masher or fork. Add the green onions and 1/2 cup Cheddar, and blend well with the potato masher, adding more of the remaining milk and butter if too dry. Season with salt and pepper.
Reduce the oven to 350 degrees F. Spoon the potatoes into a greased 1 1/2-quart baking dish and bake 15 minutes. Add the remaining 1/4 cup Cheddar on top and bake an additional 15 minutes. Serve.