01 -
Preheat oven to 350°F (175°C). Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 6–8 minutes. Let cool, then coarsely chop. Transfer to a small bowl and toss with olive oil; season with salt.
02 -
Trim root ends of leeks and 2.5 cm from the dark green tops. Cut leeks into 1.3 cm thick rounds (the dark green parts won't be perfect rounds). Rinse thoroughly to remove any grit.
03 -
Cook leeks in a large pot of boiling salted water until meltingly tender, 12–15 minutes. Transfer to a bowl of ice water to stop the cooking process. Drain and pat dry.
04 -
In a small bowl, whisk together garlic, vinegar, Dijon mustard, and whole grain mustard. Season with salt to taste.
05 -
Arrange leeks on a serving platter. Drizzle with vinaigrette and let marinate for at least 10 minutes (longer marination will enhance flavor).
06 -
Just before serving, scatter seasoned walnut mixture over leeks and top with chopped tarragon and freshly ground black pepper.