If your potatoes are already boiled and your green beans already steamed (as you indicated), bring them to room temperature or warm gently: place the halved new potatoes in a skillet over medium heat with 1 teaspoon water and a pinch of salt, and warm for 2–3 minutes to refresh their texture. Warming them lightly restores a pleasant surface texture and allows them to better absorb the vinaigrette; cold potatoes can resist dressing and make the salad taste disjointed.

If you choose to sear fresh tuna (instead of using canned), pat the 5 oz tuna dry with paper towels and season both sides with salt and pepper. Add 1 teaspoon olive oil to a heavy skillet over high heat until just shimmering. Sear the tuna 30–45 seconds per side for rare (longer for medium), turning once. Rest 2 minutes. Searing at high heat creates a thin caramelized crust that adds texture and a savory Maillard flavor without overcooking the interior — essential for a Niçoise-style tuna.

If using canned tuna, drain thoroughly and flake gently with a fork into large chunks. Canned tuna works beautifully in Niçoise when kept in large flakes rather than mashed; it gives the same satisfying bites as seared tuna without the need to cook.

Make or finish the vinaigrette: if you’re using store-bought vinaigrette, whisk 2 tablespoons with 1 teaspoon fresh lemon juice and a scant 1/2 teaspoon Dijon (optional) to brighten and bind the dressing. If making quick vinaigrette from scratch, whisk together 2 tbsp extra-virgin olive oil, 1 tbsp red wine vinegar, 1 tsp Dijon mustard, 1 tsp lemon juice, the minced 1/2 shallot, and a pinch of salt and pepper. Whisking briefly emulsifies the oil and acid so the dressing clings to potatoes and greens instead of running off.

Toss the warm potatoes with about 1 tablespoon of the vinaigrette, 1/2 tablespoon capers (if using) and a pinch of salt. Dressing warm potatoes first allows them to soak up flavor, making each bite savory and well-seasoned rather than bland.

Arrange the mixed greens on a large salad plate or shallow bowl as the base. The greens act as a cool, peppery counterpoint to the warm potatoes and savory tuna, so spread them evenly to create a balanced foundation.

Neatly arrange the dressed potatoes in one section of the plate, the steamed green beans in another, cherry tomatoes in a cluster, and olives beside them. This composed plating — classic Niçoise presentation — keeps textures distinct and showcases each ingredient's color and flavor.

Top the salad with large chunks or slices of tuna, either gently placed (for seared steaks) or flaked in generous pieces (for canned). Finish with a light drizzle (about 1 tablespoon) of remaining vinaigrette over the tuna and vegetables, and a final crack of black pepper. Adding dressing sparingly at the end preserves the textures and prevents the greens from wilting.

Finish with an optional flourish: scatter a few additional capers, a tiny grind of lemon zest, and a pinch of flaky sea salt over the tuna. These little touches amplify brightness and contrast — exactly what elevates a simple salad to restaurant quality.

Serve immediately. Advise guests to combine elements on their fork — potato, bean, olive, tomato and a piece of tuna — to experience the classic layered flavors of Niçoise in each bite.