Prepare the vinaigrette: In a jar, combine olive oil, lemon juice or vinegar, Dijon mustard, shallots, basil, thyme, and oregano. Secure the lid and shake vigorously until everything is well combined. Season with salt and pepper to your taste.

Soften the onions in vinaigrette: Place the sliced onions in a small bowl and pour 3 tablespoons of the prepared vinaigrette over them. Toss gently to coat, allowing them to marinate and mellow the sharpness.

Cook and dress the potatoes: Place the potatoes in a large pot and cover with water, ensuring they are submerged by about 2 inches. Add 1 tablespoon of salt and bring the water to a boil. Reduce the heat and simmer for 10 to 12 minutes, or until the potatoes are easily pierced with a fork. Drain the water and, while the potatoes are still warm, cut them into halves or quarters based on their size. Toss the potatoes with about 1/4 cup of the vinaigrette to coat them evenly.

Blanch the green beans: While the potatoes are cooking, fill a medium pot with water, add 2 teaspoons of salt, and bring it to a boil. Add the green beans and cook for 3 to 5 minutes, until tender but still crisp. Drain and either run them under cold water or immerse them briefly in ice water to halt the cooking process.

Assemble the salad: On a large serving platter, create a bed of lettuce. If using tuna steaks, slice them into 1/2-inch pieces. Pile the tuna in the center of the lettuce, then arrange the tomatoes, onions, potatoes, and green beans around the tuna in neat sections. Nestle the hard-boiled egg quarters, olives, and capers (and anchovies, if using) around the edges of the platter.

Finish and serve: Drizzle the remaining vinaigrette over the entire salad, ensuring each ingredient gets a little bit of dressing. Serve the salad slightly warm or at room temperature for the best flavor.