Place the baby potatoes in a saucepan and cover with cold water by a few centimetres. Salt the water well and bring to a boil. Reduce to a steady simmer and cook for 12–18 minutes, depending on size, until the potatoes are just tender when pierced with a knife. Drain and let them steam dry for a few minutes. When cool enough to handle, halve or quarter the larger ones. The potatoes should be fully tender but not waterlogged or falling apart. A waxy potato that holds clean edges is the target. That brief steam-dry period lets residual surface moisture evaporate before the potatoes are handled or dressed, protecting them from turning waterlogged.

Bring a saucepan of water to a boil. Lower in the eggs carefully and cook for 7½–8 minutes for jammy centres. Transfer immediately to ice water and cool completely. Peel and halve. The yolks should be softly jammy and deep golden, not runny and not chalky. That immediate ice bath does genuine, active work beyond simple cooling — it halts the carryover cooking that would otherwise continue firming the yolk past its intended jammy point.

Bring a pot of well-salted water to a boil. Add the haricots verts and cook for 2–3 minutes until bright green and just tender-crisp. Transfer immediately to ice water, then drain very well. They should still have bite and stay vivid green. Overcooked grey beans ruin the structure of a Niçoise. That immediate ice bath halts the cooking process at exactly the point the beans are tender-crisp and vividly green — without it, residual heat would continue softening them and dulling that colour well past the intended texture.

Whisk together the red wine vinegar, lemon juice, Dijon, garlic, salt, and black pepper. Slowly whisk in the olive oil until emulsified. Taste and adjust. The vinaigrette should be bright, sharp, and savoury enough to dress potatoes, vegetables, and tuna. That deliberate sharpness is a genuine, functional requirement given how many distinct components this composed salad carries — a milder vinaigrette would struggle to tie together potatoes, tender greens, and rich, oily tuna all at once.

Pat the tuna steaks dry very thoroughly. Rub lightly with olive oil and season all over with salt and black pepper. Heat a skillet or grill pan over high heat until very hot. Sear the tuna for about 45–60 seconds per side for rare, or slightly longer if the steaks are very thick. The outside should take colour while the centre stays deep pink-red. Transfer to a board and rest for 3–5 minutes, then slice thickly. This salad wants seared rare tuna, not fully cooked tuna. The centre should stay tender and ruby-red. Drying the tuna thoroughly before it ever meets the hot pan is what allows that brief sear to actually build real colour on the surface — any residual moisture would convert to steam and prevent the direct contact real searing depends on, especially within such a genuinely short cook time.

Place the warm potatoes in a bowl and spoon over a little vinaigrette while they are still slightly warm so they absorb it better. Lightly season the tomatoes and haricots verts with a little vinaigrette as well. Keep the greens only lightly dressed or undressed until final assembly so they stay crisp. That warmth genuinely matters for the potatoes specifically — warm potato flesh is more porous and receptive to absorbing the vinaigrette than a fully cooled one, letting the dressing actually penetrate rather than simply coat the surface.

Arrange the greens in a large wide bowl or platter. Compose the salad in neat sections over the greens: seared sliced tuna, halved jammy eggs, dressed baby potatoes, haricots verts, cherry tomatoes, Niçoise olives, anchovy fillets, red onion if using, and capers if using. Drizzle the remaining vinaigrette over the composed salad or serve some on the side. Finish with a little black pepper and parsley if desired. Arranging every component in distinct, visible sections rather than tossing everything together is what gives a proper Niçoise its whole classical identity — each element stays recognisable in its own right rather than blending into an undifferentiated mix.