Cook potatoes in large pot of boiling water until very tender, about 18 minutes. Drain in colander. Return potatoes to pot and mash with potato masher. While potatoes are cooking, heat butter in 10-inch skillet until melted. Add mushrooms and onions and cook until tender, about 3 to 4 minutes. Stir cheeses into hot potatoes until smooth. Add milk to make a creamy consistency. Stir in mushroom mixture. Place in serving bowl and serve immediately. Sprinkle with additional grated cheese, if desired.

Photo and Recipe Courtesy of the American Dairy Association (I Love Cheese).