• Place the potatoes in a large saucepan, cover with cold water and bring to a boil. Cook until tender when pierced with a knife, 12 to 15 minutes. Drain and allow to cool before cutting into bite-sized pieces.

Prepare the dressing

• While the potatoes cook, prepare the dressing. Combine the lemon juice, lemon zest, mustard, garlic, and shallot in a small bowl. Slowly whisk in the olive oil until the dressing is blended and emulsified. Season to taste with salt and pepper and set aside.

Blanch the beans

• Bring a pot of salted water to a boil for the haricot vert and set aside a large bowl of ice water. Drop the beans into the boiling water and cook until they are bright green and crisp-tender, 2 to 3 minutes (conventional green beans may take longer).
• Drain and plunge them into the bowl of ice water to stop the cooking. Drain on a clean kitchen towel and set aside.

Prepare the tuna

• Spread a small amount of the prepared dressing (a scant teaspoon) on both sides of the tuna steaks.
• Heat a frying pan over medium-high heat. Add 2 of the tuna steaks to the pan and, depending on how well done you like your tuna, sear for 1-1/2 to 2-1/2 minutes per side. Transfer the tuna to a cutting board to rest and repeat the process with the other 2 steaks.

Compose the salads

• While the tuna rests, compose the salads. Divide the lettuce between 4 serving plates, then arrange the potatoes, hard-cooked eggs, beans, tomatoes, olives, and anchovies (if using) around the outer edges.
• Slice the tuna steaks, arrange them in the center of the plates, then spoon some dressing over each serving. Top with a few capers and a little freshly ground black pepper if desired.