Put potatoes in a large pot of salted water; bring to a boil. Cook until just tender, 20 to 25 minutes. Drain; let cool.
Stir together mayonnaise, lemon juice, mustard, and dill, season with salt and pepper. Peel potatoes; cut into 1-inch chunks. Fold into mayonnaise mixture. Fold in celery, onion, scallions, and eggs (reserving some scallions for garnish). Refrigerate, covered, until ready to serve.
