Peel potatoes and halve lengthwise. Cut lengthwise into 3/4-inch-thick wedges, then cut wedges crosswise into 1-inch pieces. Steam potatoes in a steamer over boiling water in a large pot, covered, until just tender, 10 to 12 minutes.
While potatoes steam, whisk together mustard, vinegar, and salt in a large bowl, then add oil in a slow stream, whisking until emulsified.
Add hot potatoes to dressing and gently toss to combine well. Cool salad to room temperature, about 15 minutes. Add scallions and salt to taste and gently toss.
