Bring a medium pot of salted water to a gentle boil for the potatoes. Salt the water generously (it should taste like the sea) so the potatoes absorb seasoning during cooking—this is the easiest way to flavor them through.

Cook the fingerling potatoes whole until just tender, about 12–15 minutes depending on size. Test with a paring knife: it should slide in with slight resistance. Cooking the potatoes whole keeps their skins intact and prevents them from over-absorbing water; when halved and warmed later they stay creamy inside.

While the potatoes cook, prepare an ice bath (large bowl with ice and water). Blanch the trimmed green beans in the boiling potato water for 2–3 minutes until bright green and tender-crisp—this preserves color and texture. Immediately transfer to the ice bath to stop cooking and set the color. Drain and pat dry; keep warm by tossing briefly with a teaspoon of olive oil and a pinch of salt.

Make the Niçoise-style dressing: in a small bowl whisk 4 tbsp extra-virgin olive oil, 2 tbsp fresh lemon juice, 2 tsp Dijon mustard, the smashed garlic clove (or omit if you prefer a milder dressing), a pinch of salt and several grinds of black pepper. Whisk until emulsified. If using shallot, fold it in now. Taste and correct the balance: brighter with more lemon, sharper with more mustard, rounder with a pinch of sugar if needed. Preparing the dressing early lets the flavors marry.

When potatoes are tender, drain and let cool slightly then halve lengthwise. In a large skillet over medium heat melt 1 tbsp unsalted butter with 1 tsp olive oil and sear the cut side of the potatoes cut-side down until they pick up a light golden edge—about 2–3 minutes. This light browning adds a nutty, caramelized note and a pleasant textural contrast to the warm-inside potatoes. Season with salt and keep warm in a low oven (90–100°C / 200°F) or cover with foil.

Pat the tuna steaks very dry with paper towels and season both sides with salt and pepper. Drying is crucial: moisture on the surface will steam the fish and prevent a clean sear. Let the tuna sit at room temperature for 5–10 minutes so it cooks evenly.

Heat a heavy skillet (cast iron preferred) over high heat until shimmering. Add 1 tbsp neutral oil and heat until it just begins to smoke. Sear tuna 1–1.5 minutes per side for a 1-inch-thick steak for a rare center (adjust time slightly if steaks are thicker). High heat creates a thin caramelized crust quickly while leaving the interior rare and buttery—this contrast is the hallmark of seared ahi. Do not overcook; tuna becomes dry and mealy when fully cooked through.

Transfer the seared tuna to a cutting board and rest 2 minutes (resting lets residual heat equalize so the interior stays tender). Slice tuna into 6–8 mm (¼–⅓ inch) slices across the grain for attractive presentation and easy eating.

Assemble the salads family-style or plated: divide baby greens among four plates. Toss the greens lightly with about 1 tsp dressing per plate (reserve most dressing for the potatoes and tuna). Place warm halved fingerlings alongside, add the blanched green beans, scatter halved olives and capers, and arrange sliced tuna on top. Drizzle remaining dressing over the warm potatoes and tuna so they soak up the vinaigrette, enhancing flavor and texture.

Finish each plate with a final grind of black pepper and a couple of flaky sea-salt pinches over the tuna and potatoes. Serve immediately so the contrast of warm potatoes and beans with cool peppery greens and rare tuna is at its best.