Gorgeous and robust salad containing little colorful grilled potatoes and leeks, mixed in a herby mustard vinaigrette

Scale

For the Potatoes and Leeks

For the Herb Mustard Vinaigrette

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Bring a large pot of salted water to a boil. Add the potatoes and peppercorns, cook over medium heat for 20 minutes, or until fork-tender. Strain water from potatoes, discarding the peppercorns. Transfer potatoes to a baking sheet and let cool. Once cool, slice potatoes in halve lengthwise and toss in the remaining 1/4 cup olive oil and 2 teaspoons salt.
Meanwhile, make the herb mustard vinaigrette. In a medium bowl, add the mustard, vinegar, 1/2 cup olive oil, salt, scallions, parsley, and cilantro. Whisk together until blended. Set aside.
Slice off the root ends and dark green tops of the leeks and discard. Halve the leeks lengthwise, then peel back the layers by the base so they are slightly separated. Immerse the leeks in a bowl of cold water and gently shake to loosen any dirt. Pat dry. Drizzle leaks with 3 table spoons of the olive oil and sprinkle with salt.
Prepare either an indoor or outdoor grill. Grill potatoes cut-side down, until dark grill marks appear, 3 to 4 minutes, then flip and grill for 3 minutes more.
Grill the leeks cut-side down until dark grill marks form and 4 minutes, then flip and cook until the leeks no longer feel stiff, about 4 minutes more. Transfer to a plate and let cool.
In a large bowl, toss the potatoes, leeks, and herb mustard vinaigrette, move to platter, garnish with the water melon radish slices. Salad can be served right away or allowed to sit on the counter for 2 to 3 hours before serving.