Microwave potatoes in large microwaveable bowl on HIGH 18 to 20 min. or just until tender, stirring after 10 min.
Meanwhile, mix ketchup, dressing and dry seasonings until blended.
Heat oil in large saucepan on high heat. Add potatoes; cook 10 to 12 min. or until golden brown, stirring occasionally. Add ketchup mixture; cook and stir 1 min. Spoon onto platter.
Spoon mayo into small resealable plastic bag; cut small piece off one bottom corner of bag. Use to drizzle mayo over potato mixture. Sprinkle with onions.