Salade Niçoise

1/4 pound green bean s, halved

2 tablespoons red- or white- wine vinegar

1 tablespoon Dijon mustard

1/2 teaspoon salt

1/2 teaspoon fresh-ground black pepper

1/2 cup olive oil

1 head romaine lettuce (about 1 1/4 pounds), torn into bite-size pieces (about 3 quarts)

4 hard-cooked egg s, peeled and quartered

1 2-ounce tin anchovy fillets, drained

2 tablespoons drained caper s

1 6-ounce can tuna , drained

1/4 cup drained diced pimientos (from a 4-ounce jar)

1/4 cup pitted and chopped black olive s, such as Niçoise or Kalamata