Bring a medium pot of salted water to a boil over medium heat. Add potatoes and cook for 8 minutes or until slightly tender. Strain, place in large bowl and let cool until slightly warm.
While potatoes are cooking, set up grill or grill pan for direct cooking over medium heat. Place scallions on hot oiled grill and cook for 4 minutes. Transfer to a cutting board and set aside.
To the bowl with the potatoes add mayonnaise, sour cream, mustard, garlic, salt and pepper and gently mix until well blended and potatoes are coated. Slice grilled scallions and mix into the potatoes. Serve immediately or keep covered in the refrigerator for up to 2 days.