Place potatoes in a medium saucepan, and cover with water; bring to a boil. Reduce heat, and simmer 12 minutes or until tender. Add shrimp; cook 2 minutes or until done. Drain and rinse with cold water; drain.
Combine mayonnaise and the next 6 ingredients (mayonnaise through pepper) in a large bowl, stirring mixture with a whisk. Add potatoes, shrimp, and onion; toss to coat. Serve over greens.