2 tablespoons (1/4 stick) butter
chopped fresh parsley
Boil or steam potatoes and green beans separately until tender but not mushy. Drain or remove from steamer. Cut each potato into eighths. Place in salad bowl. Add green beans. Blend Dijon mustard and vinegar in small bowl. Whisk in vegetable oil in thin stream. Pour over potatoes and beans and mix gently to coat. Season salad to taste with salt and pepper. Set aside.
Melt 2 tablespoons butter in heavy large skillet over medium heat. Add leeks and sauté until tender and lightly browned, about 7 minutes.
Divide potato and green bean mixture among salad plates. Top each with sautéed leeks. Sprinkle salads with chopped fresh parsley. Garnish each salad with 2 hard-boiled egg quarters, if desired, and serve.