In 3-quart saucepan, place potatoes and enough water to cover. Heat to boiling. Reduce heat to medium; cover and simmer 15 to 20 minutes or until tender. Drain potatoes.
In same saucepan or large bowl, mash hot potatoes. Beat in yogurt, cheese, milk, salt and pepper with electric mixer on low speed until light and fluffy. Stir in onions.