Gather the ingredients.

Place potatoes in a large pot and cover with lightly 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, butter, egg, garlic, and salt in a bowl; add to potatoes. Mash until smooth; transfer to the prepared baking dish. Sprinkle with Cheddar cheese and remaining green onions.

Bake in the preheated oven until lightly browned, about 40 minutes.