Fill a pot with water and add eggs and whole potatoes. Boil together for 10-12 minutes for hard boiled eggs with set yolks. Remove eggs to ice bath immediately. Continue boiling potatoes until fork-tender at 12-15 minutes total. Drain and set aside.
Drop trimmed green beans into boiling salted water for exactly 3 minutes. Drain and immediately plunge into ice water for 2 minutes to stop cooking and retain bright green color. Drain well and pat dry.
While vegetables cool, whisk together olive oil, red wine vinegar or lemon juice, Dijon mustard, salt, and pepper in a bowl for 1 minute until thick and emulsified. Taste and adjust acidity or salt as needed.
Cut cooled potatoes into quarters or thick slices. Peel and halve hard-boiled eggs. Slice tomatoes into wedges and pat dry if watery. Thinly slice red onion or shallots.
Arrange green beans, potatoes, tomatoes, eggs, tuna chunks, olives, and sliced onion on a platter in sections without tossing. Drizzle vinaigrette evenly over the salad just before serving. Scatter chopped fresh herbs on top.
