Preheat the oven to 210C/415F/Gas 7.
For the salad, heat an ovenproof frying pan until hot. Rub the tuna loin with olive oil, then sear the tuna on each side.
Place in the oven for 5-6 minutes, or until just cooked, but still pink in the centre.
Bring a small pan of water to the boil and place the eggs into the water. Boil for 4-5 minutes, then drain and run under cold water to stop them continuing to cook. Peel off the shell.
Bring another pan of salted water to the boil, add the French beans and cook for 3-4 minutes, until just tender. Drain and refresh under cold water, then slice in half lengthways.
Place the cherry tomatoes onto a baking sheet and drizzle with a little olive oil. Place in the oven for 3-4 minutes, until just softened.
For the dressing, place the garlic and anchovy fillets into a pestle and mortar with the egg yolk and pound to a paste.
Add the lemon juice and stir, then gradually add the olive oil in a slow drizzle, mixing all the time with a whisk.
Stir in the mustard and season, to taste, with salt and freshly ground black pepper.
To assemble the salad, place the beans, tomatoes, potatoes, olives, eggs and lettuce leaves into a bowl. Drizzle over the dressing and stir well.
To serve, divide the salad among four serving plates. Cut the cooked tuna loin into thin slices and place on top of the salad.