Cook the potatoes and beans: Cook the potatoes in a pan of lightly salted boiling water for 10-12 minutes or until tender. Add the French beans for the last 4 minutes of cooking. Drain, rinse under cold water and drain again.
Boil the eggs: While the potatoes are cooking, add the eggs to a pan of boiling water and cook for 7-8 minutes, so that the yolks remain slightly soft and creamy. Drain the eggs and put them into a bowl of cold water. Peel and cut into quarters lengthways.
Make the dressing: Drain the can of anchovy fillets. Put the anchovies, garlic, mustard, lemon juice and olive oil into a blender or food processor and purée until smooth and creamy. Spoon half of this dressing over the cooked potatoes and beans, and gently toss together.
Assemble the salad: Separate the tuna into chunks. Put the lettuce into a salad bowl. Top with the potatoes and beans followed by the eggs and tomatoes, then scatter over the tuna and olives. Serve drizzled with the remaining dressing.
*For tuna, use 200g cooked, peeled prawns. *For French beans, use 200g asparagus spears. Snap off any woody ends from the asparagus stalks at the point where they break easily, then cut the spears into short pieces and steam over the potatoes for 4-5 minutes.
So that the potatoes cook as quickly as possible, cut any larger ones in half to make sure they are all roughly the same size.
The dressing can be made the day before and stored in a covered jug or bowl in the fridge. Whisk with a fork before using.