In a large saucepan cook potatoes, covered, in enough boiling salted water to cover for 10 to 12 minutes or until tender. Drain; rinse with cold water. Drain again.
Meahwhile, in a large bowl stir together the soup, sour cream, cream cheese, and butter. Stir in 1/4 cup of the shredded cheese, 3 tablespoons of the green onions, the milk, garlic salt, and pepper. Stir in the cooked potatoes. Transfer mixture to a 2-quart rectangular baking dish.
Bake in a 350 degree F oven for 25 to 30 minutes or until heated through. Sprinkle with the remaining 1/2 cup shredded cheese. Let stand about 5 minutes or until cheese melts. Sprinkle with the remaining 1 tablespoon green onions. Makes 10 servings.