FINGERLING POTATO SALAD
3 1/2 pounds fingerling potatoes, unpeeled, cut into 3/4"-1" pieces
1 tablespoon kosher salt plus more for seasoning
9 tablespoons (or more) extra-virgin olive oil, divided
2 tablespoons brown mustard seeds
3 medium leeks (white and palegreen parts only), halved lengthwise, cut crosswise into ¼" slices (about 5 cups)
1 tablespoon (or more) white wine vinegar
1 tablespoon Dijon mustard
Freshly ground black pepper
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.