Cook the potatoes in a saucepan of boiling water for 15 minutes or until just tender. Use a slotted spoon to transfer the potatoes to a chopping board. Halve. Add the beans to the pan and cook for 2-3 minutes or until bright green and tender crisp. Refresh under cold running water. Drain.
Meanwhile, place eggs in a saucepan of cold water. Bring to the boil over high heat.
Reduce heat to medium and gently boil, uncovered, stirring occasionally, for 6-7
minutes. Drain and cool under cold running water. Peel and quarter.
Whisk together the oil, vinegar, mustard and sugar in a jug until combined. Season with salt and pepper.
Heat a frying pan over medium-high heat. Add tuna and cook for 2-3 minutes each side for medium or until cooked to your liking. Transfer to a plate. Set aside for 5 minutes to rest. Flake the tuna into large pieces.
Divide the potato, beans, egg, tuna, tomato, olives and parsley evenly among serving bowls. Drizzle over the dressing. Season with salt and pepper to serve.