Peel potatoes; cut into 1-inch cubes. Cook in boiling water to cover 15 minutes or until tender. Drain.
Mash potato with a potato masher; stir in milk and next 5 ingredients until blended. Stir in cheese, green onions, and, if desired, ham. Garnish, if desired.
*1/2 cup shredded Swiss cheese may be substituted.