How to Make It
Place potatoes, 8 cups water, and 2 tablespoons of the salt in a large saucepan, and bring to a boil over high. Reduce heat to medium. Simmer, uncovered, until potatoes are fork-tender, 12 to 14 minutes. Drain and rinse with cold water until potatoes are room temperature, and transfer to a large bowl.
While potatoes boil, slice scallions into 1/2-inch pieces, keeping green and white parts separate. Set aside the green parts. Place white parts in a small microwavable bowl; add red onion, apple cider vinegar, and 1/2 teaspoon of the salt. Microwave on HIGH for 90 seconds. Stir mixture; press down on onion slices to submerge in vinegar. Let stand at room temperature 15 minutes; drain.
Whisk together oil, whole-grain mustard, lemon juice, Dijon mustard, honey, and remaining 1 1/2 teaspoons salt in a large bowl until blended. Add potatoes; toss to coat. Transfer mixture to a platter; sprinkle with scallion greens and pickled onion mixture.