Fingerling Potato Salad

INGREDIENTS

• 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