2 lb small potatoes (or larger ones chopped into 1" pieces)1+1/2 tsp. salt 3 Tbsp. finely chopped onions or shallots 1/2 Tbsp. Dijon mustard 1 Tbsp. whole-grain mustard 1 Tbsp. white-wine vinegar, or to taste 1/2 tsp. black pepper 3 Tbsp. olive oil 2 Tbsp. chopped fresh flat-leaf parsley
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 onion (or 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.
adapted from recipe found in Gourmet, August 2006. from epicurious.com