Salad Niçoise

Bring a large pan of water to the boil. Add the potatoes and the eggs, and cook for 7 mins. Scoop the eggs out of the pan, tip in the green beans and cook for a further 4 mins.
Drain the potatoes, beans and eggs in a colander under cold running water until cool. Leave to dry.
Peel the eggs and cut into quarters. Arrange the lettuce leaves in 2 shallow bowls. Scatter over the beans, potatoes, tomatoes and egg quarters. Pat the anchovies with kitchen paper to absorb the excess oil and place on top.
Flake the tuna into chunks and scatter over the salad. Mix the mayonnaise and 1 tbsp cold water in a bowl until smooth. Drizzle over the salad and serve.