Fill a large bowl with cold water and ice cubes.
Steam string beans and potatoes for 12 to 15 minutes, until potatoes are fork tender. Drain vegetables and immediately plunge them into the ice water. Drain again.
While the vegetables are cooking, heat a large sauté pan over high heat.
Brush tuna with reserved marinade. Sear tuna 3 minutes on each side for medium-rare, or until desired doneness. Let cool slightly and cut into eight equal slices.
To assemble salad: Place 2 lettuce leaves on each of 4 plates. Distribute tuna, string beans, potatoes, artichoke hearts, onions, olives, eggs, tomatoes, and white beans among the plates.
To make dressing: In a small bowl, whisk together vinegar, lemon juice, garlic, and mustard. Slowly whisk in oil; season with salt and pepper.
Drizzle dressing over salad and serve with crusty bread