Put a rack in the middle of oven and preheat oven to 400 degrees F.
Cut off and discard top quarter of garlic head. Place in a piece of foil and drizzle with just a tiny amount of olive oil (about 1/8 teaspoon). Bring up all sides of foil and twist tightly at the top to wrap the garlic tightly in the foil. Scrub potatoes and prick with a fork. Bake potatoes and garlic on oven rack for 45 minutes.
Remove garlic and let cool. If crispy skin is desired, rub potatoes with olive oil and cover with foil; continue baking potatoes until tender, about 20 minutes more. Take potatoes out of oven, but leave the oven on.
Squeeze pulp from garlic cloves into a medium bowl and discard skin. Stir in butter, sour cream, and 1 cup Cheddar. Cutting lengthwise, slice off top quarter of each potato and discard. Leaving 1/4-inch-thick shells, scoop flesh out of potatoes and add to cheese mixture. Mash with fork to combine. Season with salt and pepper and divide among shells.
Arrange potatoes in a small baking pan and sprinkle with remaining 1/2 cup Cheddar. Bake until heated through, 15-20 minutes. Add a dollop of sour cream and a generous sprinkling of chopped green onions on top before serving.