Combine olive oil, vinegar, and Dijon mustard in a small bowl. Season with salt and pepper to taste.
Peel the eggs and cut them in half. Arrange the lettuce on top of a salad plate. Add the halved cooked potatoes, cherry tomatoes, string beans, chopped cucumber, and thinly sliced red onion. Arrange the halved hard-boiled eggs on top and add the tuna. Drizzle with the salad dressing, sprinkle with capers and serve.