2 1/2 lbs sweet potatoes (2 large)
1 tablespoon Dijon mustard
4 tsps white wine vinegar
1/4 teaspoon salt
1/4 cup olive oil
4 scallions thinly sliced
Peel potatoes and halve lengthwise. Cut lengthwise into 3/4" thick wedges; then cut wedges crosswise into 1" pieces. Steam potatoes over boiling water, covered, until just tender -- 10-12 minutes.
While potatoes steam, whisk together mustard, vinegar & salt in a large bowl. Add oil in a slow stream, whisking until emulsified.
Add hot potatoes to dressing and gently toss to combine well. Cool to room temperature, about 15 minutes. Add scallions and salt to taste and gently toss.
Salad with out scallions can be made 2 hours ahead. Toss in scallions just before serving.