Tuna Niçoise Salad

Place a steamer basket in the bottom of a large pot and add water to just below the bottom of the basket. Cover and bring to a boil over high heat. Add potatoes to the basket and reduce heat to medium; cook, covered, until tender, about 15 minutes. (Do not remove pot from heat.) Transfer potatoes to a plate and let cool for about 10 minutes. 

Meanwhile, add green beans to the basket; cook over medium heat, covered, until tender-crisp, about 5 minutes. Transfer the beans to the plate with the potatoes. Cut the cooled potatoes in half crosswise.

Whisk lemon juice, shallot, mustard and 1/4 teaspoon each salt and pepper in a medium bowl until smooth. Whisking constantly, gradually drizzle in 4 tablespoons oil. Whisk in parsley until combined.

Pat tuna dry; sprinkle with the remaining 1/4 teaspoon each salt and pepper. Heat the remaining 1 tablespoon oil in a medium nonstick skillet over medium heat. Add the tuna and cook, turning once, until lightly browned, about 2 minutes per side. Transfer to a clean cutting board and let rest for 5 minutes. Slice 1/2-inch thick against the grain. 

Arrange lettuce on a platter or 4 plates; top with the sliced tuna, halved potatoes, green beans, eggs, tomatoes and olives. Drizzle evenly with the dressing and serve immediately.