Place potatoes in a large pot and cover with water. Bring to a boil and cook for about 20 minutes; the potatoes are done when a knife is easily inserted into the potato. Drain and cool potatoes in the refrigerator for approximately 20 minutes
In a large bowl, combine the lemon zest, lemon juice, and mustard. Season, to taste, with salt and pepper. Slowly whisk in the olive oil. Add red onion, red pepper, parsley, and scallions and stir to combine and coat with the dressing. Season, to taste, with salt and pepper.
Remove potatoes from refrigerator cut into eighths or bite-size pieces. Add potatoes to bowl and toss to combine. Adjust seasonings, if necessary.