Quarter the potatoes or cut into 1/2 inch dice. No need to peal the potatoes first, but if you like the extra work, go ahead.
Bring a pot of water to a boil. Add a TSP of salt and the potatoes. Simmer for about 12-15 minutes.
While the potatoes simmer, trim the ends from the green beans.
After the potatoes have simmered for 12-15 m inutes, add the eggs to the pot, using a large spoon so they don't crack. Put a steamer basket on top of the potatoes and eggs, add the green beans, cover, and simmer for 5-7 more minutes. Using one pot cuts down on the cleanup, although I suppose you could use two or even three if you like cleaning pots. While the potatoes, eggs, and green beans cook, place a skillet over medium-high heat and add 1 TBSP of the olive oil to the pan. Brush the tuna steaks with the remaining olive oil, and brush with a bit of the dressing. Salt and pepper to taste and add to hot pan. Cook each side for two minutes. The tuna will be bright red in the middle with a seared exterior.
When the potatoes, eggs, and green beans are done cooking, remove the steamer basket and set it and the beans aside. Remove the eggs from the pan, and drain the potatoes in a colander. Fill the pan with cold water and put the eggs back in--this will make it easier to peal them.
When the tuna is done, remove from pan and cut into slices against the grain of the meat.
Peal and slice the eggs. Alternatively, just cut the eggs in half, but slicing them looks better and is easier to eat.
Dice the green onions, if using.
Arrange the salad on the plate on a bed of greens. Drizzle with the dressing. Garnish with the green onions.This is a composed salad, so traditionally, you would arrange each ingredient separately, but at a minimum the tuna, eggs, and anchovies (if used) should be kept separate. You could combine the green beans, tomatoes, capers, olives, and potatoes in a bowl and mix together with the dressing, creating a kind of "potato salad" for a slightly different presentation as shown in the photo above.