To make the vinaigrette: combine all ingredients in a mason jar. Screw the cap on the jar and shake the vinaigrette vigorously to emulsify. Set the dressing aside while preparing the salad so the flavors can marry.
Cooking the potatoes, eggs, and green beans in the same pot cuts down on prep time and clean up. To do this, put the potatoes in a large saucepan, add water to cover and a nice pinch of salt; bring to a boil over medium heat. Simmer the potatoes for 12 minutes to give them a head start, and then add the eggs. Place a steamer basket or colander on top of the simmering water. Put the green beans in the steamer and cover with a lid. Steam the beans for 5 minutes until crisp-tender while continuing to cook the potatoes until fork tender. Drain out the water and put the potatoes, eggs, and green beans in a colander; rinse briefly under cold water. Peel the shells off the eggs and cut them in 1/2 lengthwise.
Place a large skillet over medium-high heat. Rub the tuna on all sides with olive oil, and a bit of the vinaigrette; season with a fair amount of salt and pepper. Lay the tuna in the hot pan and sear for approximately 2 minutes on each side; as the tuna cooks, the red meat will become whiter. Transfer the tuna to a cutting board and slice.
To assemble the salad: combine the potatoes, green beans, tomatoes, olives, anchovies, capers, and chives in a large mixing bowl. Take the vinaigrette and give it another good shake to recombine. Drizzle the salad with enough vinaigrette to fully moisten and toss gently to coat; season with salt and pepper. Take care not to mush up the ingredients - the important thing about salad nicoise is that it is arranged nicely on a platter with all the elements keeping their individual integrity. Put the tossed salad down the center of a serving platter and lay the seared tuna attractively across the top and the eggs around the rim. Drizzle with the remaining vinaigrette and serve.