Cook the potatoes in a saucepan of lightly salted boiling water for 12 minutes. Add the beans and cook for a further 3–4 minutes until both vegetables are tender. Drain and briefly refresh under cold running water. Cut the potatoes in half (if you like, you can also peel them), then transfer the potatoes and beans to a large bowl.
Meanwhile, put the eggs in a small saucepan of water and bring to the boil. Cook for 6–8 minutes, depending how hard you like your eggs. Drain, then refresh under cold running water. When cool enough to handle, peel the eggs, then cut into quarters.
To make the dressing, combine the garlic, mustard, oil, vinegar and salt and freshly ground black pepper in a small bowl and whisk together. (Alternatively, put all the ingredients in a screw-top jar, then shake well until combined.) Pour about two-thirds of the dressing over the warm vegetables and toss to coat.
Brush the tuna steaks with olive oil and lightly season with pepper. Heat a cast-iron chargrill pan or non-stick frying pan over a moderately high heat and cook the tuna steaks for 3–4 minutes on each side until just tender. Remove from the pan and set aside.
Separate the lettuce leaves, then tear into bite-sized pieces. Put the leaves in a shallow bowl or serving platter or divide among four individual plates.
Add the onion, cucumber, tomatoes and anchovies to the potato and bean mixture, gently toss together, then spoon on top of the lettuce. Arrange the egg quarters around the edge of the dish.
Cut the tuna steaks into strips and arrange on top of the salad. Scatter the olives over the top, then drizzle with the remaining dressing and serve.