Place potatoes in a large pot and cover with salted water; bring to the boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and return to pot.
Preheat the oven to 190 C / Gas 5. Lightly grease a baking dish.
Combine milk, cream cheese, 2/3 of the spring onions, egg, butter, garlic and salt together in a bowl. Add milk mixture to potatoes in the pot; mash together until mixture is smooth. Spoon into the prepared baking dish. Sprinkle with Cheddar cheese and remaining sliced spring onions.
Bake in the preheated oven until lightly browned, about 40 minutes.