1. Combine all dressing ingredients in glass measuring cup. Stir with fork to blend well; set aside.
2. Prepare grill for direct cooking. Bring 8 cups water and salt to a boil. Cut potatoes into 1/2-inch slices. Add potatoes to water; boil about 5 minutes. Drain into colander. Transfer potatoes to bowl. Drizzle with oil; toss lightly.
3. Spray 1 side of foil with cooking spray. Transfer potatoes to foil; fold into packet. Place potato packet on grid over medium-high heat. Grill 7 to 10 minutes or until potatoes are tender. Transfer potatoes to serving bowl. Sprinkle with green onion. Toss gently with reserved dressing. Serve warm.