Ingredients

• 1 1/2 pounds fingerling potatoes (such as La Ratte or Russian Banana), cut into 1/2-inch disks
• Kosher salt
• 2 tablespoons white wine vinegar, divided
• 1 tablespoon whole grain mustard
• 2 tablespoons sour cream
• 2 teaspoons extra-virgin olive oil
• 1 small red onion, diced (about 3/4 cup)
• 2 scallions, finely sliced (about 1/4 cup)
• 2 tablespoons minced fresh dill
• Freshly ground black pepper

Instructions

1. Place potatoes, 1 tablespoon salt, 1/2 tablespoon vinegar, and 3 cups tepid water in a medium saucepan. Bring to a boil over high heat, stirring occasionally until salt is dissolved. Reduce to a bare simmer and cook until potatoes are completely tender and show no resistance when poked with a paring knife or cake tester, about 17 minutes. Drain potatoes. Immediately toss potato pieces with 1 tablespoon vinegar, spread in a single layer in a rimmed baking sheet, and allow to cool to warm room temperature, about 10 minutes.
2. Meanwhile, combine mustard, sour cream, olive oil, red onion, scallions, dill, and remaining 1/2 tablespoon vinegar in a large bowl and whisk together. Add potatoes and toss thoroughly to combine. Season to taste with more salt and pepper as necessary. Serve.