In a small bowl, combine all ingredients. Whisk to combine. Set aside to allow the flavors to meld together.
Prepare the chickpeas:
In a medium bowl, combine the chickpeas, lemon juice, mustard, oregano, and salt. Stir to combine. Set aside.
Make the salad:
Fill a medium bowl with ice water. Bring a large pot of water to a boil over high heat. When the water is boiling, reduce the heat to medium-high, add the green beans, and cook until the beans are bright green and just tender, 3-4 minutes. Using a slotted spoon, scoop out the green beans and submerge them in the ice water. Add the potatoes to the boiling water and cook until fork-tender, 10-12 minutes. Drain the potatoes and rinse under cool running water.
Drain the green beans. Pat the potatoes and green beans dry.
Spread the romaine lettuce on a large serving platter. Evenly arrange the green beans, potatoes, tomatoes, cucumber, olives, and red onion over the romaine. Sprinkle with the seasoned chickpeas, chives, capers, and a pinch each of salt and pepper. Serve the dressing on the side for drizzling. (Alternatively, you can toss all the ingredients together in a large salad bowl and toss with the dressing to taste just before serving.)
The dressing can be prepared up to 3 days in advance. Store in a jar in the refrigerator. Bring to room temperature and shake before using.