Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and return to pot.
Preheat the oven to 375 degrees F (190 degrees C). Grease a glass baking dish.
Combine milk, cream cheese, 1/3 cup green onions, egg, butter, garlic, and salt in a bowl. Pour onto the prepared potatoes. Mash together until mixture is smooth. Spoon into the prepared baking dish. Sprinkle with Cheddar cheese and remaining green onions.
Bake in the preheated oven until lightly browned, about 40 minutes.
You can use plain or herbed cream cheese.
Chives can be substituted for the green onions.
To make ahead, cool, cover, and refrigerate for up to 24 hours; you will need to increase baking time to 55 minutes.
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