1 lb small potatoes
1 1/2 teaspoons salt
3 tablespoons finely chopped shallots (about 2)
1/2 tablespoon Dijon mustard
1 tablespoon whole-grain mustard
1 tablespoon white-wine vinegar, or to taste
1/2 teaspoon black pepper
3 tablespoons olive oil
2 tablespoons chopped fresh flat-leaf parsley or other herbs
Cover potatoes with water by 1 inch in a 3- to 4-quart saucepan, then bring to a boil with 1 teaspoon salt. Simmer, partially covered, until potatoes are tender, about 10 minutes, then drain.
Whisk together shallot, mustards, vinegar, pepper, and remaining 1/2 teaspoon salt in a large bowl, then add oil in a slow stream, whisking until emulsified.
When potatoes are just cool enough to handle, halve them, then add to vinaigrette along with parsley and toss to combine. Serve warm or at room temperature.