Begin by bringing a large, heavy-based pan of salted water to the boil. Add the potatoes, bring back to the boil and simmer for 10 minutes, or until soft. Remove from the pan and plunge into a bowl of ice-cold water. Leave to cool slightly.
While the potatoes are boiling, bring another pan of water to the boil and add the eggs. Bring back to the boil and simmer for 6 minutes. Remove from the pan and transfer to a bowl of ice-cold water until cool enough to handle.
In the pan the potatoes were cooked in, but in clean water, boil the beans for 2-3 minutes, then remove from the pan and transfer to a bowl of ice-cold water. Leave to cool slightly.
While the potatoes, beans and eggs are cooling, cut the tomatoes into rough chunks and add to a large mixing bowl. Generously season with salt.
Halve the olives and add to the bowl with the tomatoes, plus the anchovies, cut into rough chunks, and the chopped parsley. Toss to combine.
Make the dressing by adding the mustard to a small bowl or jug and whisking with the vinegar, garlic, salt and pepper. Mix to combine, then slowly pour in the olive oil, mixing as you go until the dressing is smooth and well combined. Taste for seasoning and add more vinegar, salt or pepper if need be.
Once cooled slightly, add the potatoes and beans to the mixing bowl with the tomatoes. Break the tuna into large flakes and also add to the mixing bowl. Pour over half of the dressing and gently mix.
Plate the salad between bowls and add more dressing if desired. Peel and slice the boiled eggs and place on top of the salad in each serving bowl. Season the yolk with a pinch of salt and serve.
