Fingerling Potato Salad

Place potatoes in a large pot. Add enough cold water to cover by 3". Stir in 1 tablespoons salt and bring to a boil. Reduce heat to medium and simmer until potatoes are tender, about 10 minutes. Drain. Transfer to a large rimmed baking sheet and let cool slightly. Heat 3 tablespoons oil in a small skillet over medium-high heat. Add mustard seeds to skillet and cook, stirring occasionally, until seeds start to pop, about 2 minutes. Pour oil with seeds into a large bowl. Heat 2 tablespoons oil in a large skillet over medium heat. Add leeks, season with salt, and cook, stirring occasionally, until just tender, 10-12 minutes. Whisk remaining 4 tablespoons oil, vinegar, Dijon mustard, and 1 tablespoon water into mustard-seed oil. Add potatoes and leeks; toss to coat. Season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Cover; chill. Return to room temperature before serving, adding more oil and vinegar if dry.