1/2 small red onion, or 1 large shallot, peeled and finely diced (about 1/2 cup)
Juice of 2 lemons
1/4 teaspoon fine salt
1/4 teaspoon freshly ground black pepper
1 (24-inch-long) baguette
2 tablespoons extra-virgin olive oil
1 tablespoon red wine vinegar
2 (6-ounce) cans or jars tuna packed in olive oil (do not drain)
1/2 cup pitted Niçoise olives, coarsely chopped
1/4 cup fresh flat-leaf parsley leaves or 1/2 cup fresh basil leaves, coarsely chopped (optional)
2 teaspoons capers, drained (optional)
4 oil-packed anchovy fillets, drained (or 1 teaspoon anchovy paste)
2 large tomatoes, cored and cut into 1/2-inch slices
2 hard-boiled eggs, cut into 1/2-inch slices
8 ounces green beans or haricots verts, blanched (optional)
Place the onions, lemon juice, salt, and pepper in a medium bowl and mix gently to combine. If you are using the anchovy paste (and not the whole anchovies), add to the mixture and mix well.
Using a serrated knife, cut the baguette in half lengthwise. Place the halves cut-side up on a very large piece of plastic wrap (see Recipe Notes). Drizzle the olive oil and vinegar evenly over both cut sides of the bread.
Add the tuna and its oil to the onion mixture, and stir and mash until well-combined. Add the olives and mix well. Add the parsley or basil, and capers if using, and mix to combine. Spoon the tuna mixture evenly over the bottom piece of bread.
If you are using whole anchovies, arrange them evenly over the tuna. Arrange the tomato slices over the anchovies. Place the egg slices evenly over the tomatoes. Arrange the green beans on top, if using. Top with the other half of the baguette.
Wrap the plastic wrap securely around the sandwich and squash it down with your hands. Wrap securely in aluminum foil. Refrigerate until the sandwich is well-chilled, the bread is soft inside, and the tomatoes have released their juices but the crust is not falling apart, 2 to 4 hours. Cut the sandwich into 4 equal pieces just before serving.