Place potato in a large saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 20 minutes or until potato is tender; drain.
Combine milk and onions in a small saucepan; bring to a simmer. Cook 3 minutes. Remove from heat.
Combine potato, milk mixture, butter, salt, and pepper in a bowl; beat with a mixer at medium speed until smooth.