Trim the tough dark green tops and stem ends of 12 leeks, but keep the bases intact. Halve lengthwise and rinse well. Boil in salted water until tender, about 10 minutes. Drain and cool in a bowl of salted ice water, then drain and pat dry. Whisk 1 tablespoon dijon mustard, 2 tablespoons white wine vinegar, 1/2 teaspoon salt, and pepper to taste in a bowl. Whisk in 1/3 cup walnut oil and drizzle over the leeks. Top with chopped toasted walnuts and chopped parsley.
Photograph by Marcus Nilsson