How to Make It
Place potatoes in a small saucepan, and cover with water. Bring to a boil over high, and reduce heat to medium-low to maintain a simmer. Simmer until tender, about 12 minutes. Drain potatoes, and cool to room temperature, about 20 minutes. Cut potatoes into 1/3-inch pieces.
Bring 1 inch of water to a boil in a medium saucepan fitted with a steamer basket over high. Add haricots verts to basket; cover and steam until tender-crisp, 3 to 4 minutes. Transfer beans to a bowl of ice and water, and let stand until chilled, about 5 minutes. Drain beans on paper towels, and cut diagonally into 1/2-inch pieces.
Toss together potatoes, haricots verts, olives, egg, parsley, and chives in a medium bowl. Set aside.
Whisk together vinegar, lemon juice, shallot, mustard, salt, and pepper in a small bowl. Slowly whisk in oil until emulsified. Drizzle dressing over potato mixture; gently toss to coat. Let stand 5 minutes before serving.