Step 1

Whisk the lemon juice, mustard, shallot, mustard, salt, and pepper in a medium bowl. Whisk in the olive oil in a slow, steady stream until thick and emulsified. Set dressing aside.

Step 2

Boil the green beans in salted water for 3-4 minutes or until just cooked through, but still crisp and bright green. Drain and immediately plunge them into a bowl of ice water. Beans can be cooked a day ahead, kept in the fridge wrapped in a damp paper towel, inside a sealed plastic baggy.

Step 3

Scrub or peel potatoes and cut them into 1/4-inch slices crosswise, placing slices in a large bowl of cold water, as you go, to prevent browning. Boil potatoes in salted water for 5 minutes or until they are fork-tender. (They will soften a bit more as they cool.) Drain and transfer them to a large bowl. Season with salt, pepper, and 1 tablespoon of vinegar. Toss gently and let them cool for 10 minutes or so. Potatoes can be stored overnight in an airtight container in the fridge. (see notes below for more details and substitutions)

Step 4

Cut the eggs crosswise into thin slices or cut them in half or into quarters. Do this step at the last minute.

Step 5

Compose the salad on a large platter or arrange it onto individual plates. Line a large platter (or plates) with lettuce (trick: for Bibb lettuce, if the leaves curl too much, crack the thick white spines gently to help them lie flat). Arrange ingredients in separate piles around the platter. I like to start with the tuna - slightly off center - and work around it. Sprinkle tomatoes, green beans, eggs and tuna with little salt and pepper. Toss the potatoes with a few tablespoons of dressing, before adding them. Drizzle dressing over everything else except the olives, capers and anchovies. If there's any remaining dressing, serve it on the side. Sprinkle fresh herbs over everything and serve.