There are so many different ideas of what constitutes a Niçoise Salad salad, that it has almost become more of a category, rather than an actual defined salad. Though it’s a French salad, variations even exist throughout France.
The ingredients are not tossed together; rather this is a “composed salad” meaning the ingredients are arranged separately on the plate.
At the start of the 1900s, Niçoise Salad contained only anchovies, artichoke hearts, olives, tomatoes and peppers, tossed with a dressing made of olive oil, vinegar and mustard.
Tuna, and small boiled potatoes have been added to most recipes over the years since then, and in many instances, broad beans, green beans, chopped boiled eggs, and onion as well.
It is now usually served on a bed of lettuce.
Most people agree that the tomatoes are to be quartered, not sliced.
Julia Child insisted on green beans and tomatoes in the recipe, and on lining the bowl with lettuce. She also insisted that each ingredient be tossed in the dressing separately, and then assembled, rather than combining everything and then tossing. “I’ve seen people swish everything together,” she said. “And it just looks like hell.” 
 In: Suzy Patterson. “Vacation doesn’t mean getting away from food.” Charleston, West Virginia: Sunday Gazette-Mail. Sunday, 3 September 1978. Page 83.
One of the core ingredients in everyone’s version is tomatoes .The French didn’t start eating tomatoes until well into the 1800s.
For a time in the mid-1900s, it was almost de rigeur for any cookbook published in the English language to have a Salade Niçoise recipe in it.
Literature & Lore
“Salade Nicoise Dish Good for Lenten Season: Want an unusual Lenten dish? Try ‘Salade Nicoise’, exactly as made in Nice, France, by the noted restaurant of Da Bouttau. This 94-year-old restaurant, well known to some American travelers because it’s used as a meal stop by Pan-American World Airways, which offers this translation for “Salade Nicoise”: Line a large bowl with cut lettuce and endive leaves. Cover the greens with sliced tomatoes, chopped celery and onions and sliced green peppers. Radishes may also be added. Put in about four portions of canned tuna fish, plus a can of flat anchovies and several greek black olives (ripe olives packed in oil). In a separate dish, make a sauce to taste, using olive oil, vinegar or lemon juice, mustard, salt and pepper and a bit of minced garlic clove. Add the sauce to the salad and mix before serving.” — The Charleston Gazette. Charleston, West Virginia. Sunday, 21 March 1954. Page 7.
“Food authorities do not agree on the ingredients in Salade Nicoise. Larousse Gastronomique calls for equal parts of diced potatoes and green beans seasoned with oil and vinegar and decorated with anchovies, olives, capers and quartered tomatoes. Julia Child agrees but adds tuna. Michael Field adds hard-cooked eggs. Myra Waldo omits the potatoes and green beans, combining the other ingredients with romaine. Salade Nicoise is usually served with herbed oil and vinegar dressing, but there are variations made with mayonnaise. For instance, New York’s excellent Le Veu d’Or restaurant serves theirs with a Russian-type dressing.” — Walla Walla Union-Bulletin. Walla Walla, Washington. Wednesday, August 19 1970. Page 13.
[Ed: In 1978, Julia Child opened a tin of sardines instead of tuna while making a Niçoise Salad.] “Forgot to put on my glasses,” she said. “Never mind, it just adds a new wrinkle.” — As quoted by Suzy Patterson in “Vacation doesn’t mean getting away from food”. Charleston, West Virginia: Sunday Gazette-Mail. Sunday, 3 September 1978. Page 83.
Slater, Nigel. Nigel Slater’s classic salade Niçoise recipe. Manchester: The Observer. 8 August 2010.