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