In a jar, combine the oil, lemon juice or vinegar, shallots, herbs, and mustard. Close the lid tightly and shake well to blend. Season with salt and pepper to taste.
Place the onion slices in a small bowl and sprinkle them with 3 tablespoons of the vinaigrette. This will help mellow the flavour of the onions.
In a large pot, cover the potatoes with 2 inches of water and add 1 tablespoon of salt. Bring to a boil and simmer for 10 to 12 minutes, until the potatoes are fork-tender. Drain and cut them into halves or quarters while still warm. Place them in a bowl and toss with about 1/4 cup of the vinaigrette.
Fill a medium-sized pot halfway 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 they are tender yet firm. Drain and rinse with cold water or shock them in ice water to stop the cooking process.
On a serving platter, create a bed of lettuce. Slightly crumble the tinned tuna steaks and place them in the centre of the lettuce. Scatter the tomatoes and onions around the tuna. Arrange the potatoes and green beans over the bed of lettuce or in mounds at the edge of the lettuce. Decorate the platter with the hard-boiled eggs and olives, or use them to divide the other ingredients.
Drizzle the remaining vinaigrette over the entire salad. Optionally, sprinkle with capers for added flavour.
Salad Niçoise is best served warm or at least at room temperature. So, serve it immediately to enjoy the delightful blend of flavours.