This mashed potato casserole is a delicious combination of hot mashed potatoes, sour cream, cheddar cheese, and green onions. The baked casserole takes everyday mashed potatoes to new levels. Feel free to change the ingredients up a bit to suit your family's tastes. Add some finely chopped ham or crumbled cooked bacon for a loaded potato casserole. If you like a more subtle onion flavor, replace the green onions with a few tablespoons of fresh chopped chives. A tablespoon or two of fresh chopped parsley is another excellent option. Or, if you like a crisp topping, sprinkle about 1/2 to 1 cup of buttered breadcrumbs over the potato casserole just before you put it in the oven. French fried onions would make a tasty topping as well. This potato casserole is an excellent choice for a holiday meal, sunday dinner, or any special family meal. Or take the dish along to a potluck dinner.