Bring a large pot of salted water to a boil for the potatoes. Salted water seasons potatoes from the inside and raises the boiling point for even cooking. If your fingerlings are already boiled, skip to step 3.

Boil the fingerling potatoes (whole) for 12–15 minutes until tender when pierced with a skewer; larger ones take a bit longer. Drain, let cool slightly, then halve lengthwise. Parboiling whole fingerlings keeps their skins intact and concentrates flavor; halving exposes a cut surface you can crisp in a pan.

Blanch the green beans: add trimmed beans to boiling salted water for 2–3 minutes until bright green and just tender; immediately transfer to an ice bath to stop cooking and lock color. This preserves the beans’ snap and prevents an overly soft texture on the plate.

Make the lemon‑Dijon vinaigrette: in a small bowl whisk together 4 tbsp extra‑virgin olive oil, 4 tsp Dijon, and 4 tsp fresh lemon juice with a pinch of salt and pepper. Whisking emulsifies oil and mustard so the dressing stays cohesive and clings to salad leaves and potatoes.

Soft‑boil the eggs: bring a small saucepan of water to a gentle boil, lower 4 room‑temperature eggs with a slotted spoon, and cook for 6 minutes for a set white and jammy yolk. Transfer to an ice bath for 3 minutes, then peel carefully. Timing is key—6 minutes gives a ripe, spoonable yolk that finishes the Niçoise beautifully.

Warm a large skillet over medium-high heat and add 1 tbsp olive oil. When the oil shimmers (it should just begin to smoke), season the flank steak generously with salt and freshly ground black pepper. High heat creates a Maillard crust that adds deep savory flavor.

Sear the flank steak: place the steak in the hot pan and sear without moving for 2.5–3 minutes per side for medium-rare (internal temperature ~57–60°C / 135–140°F). If your steak is thicker, add 30–60 seconds per side. Searing seals and develops flavor; brief cooking keeps flank tender. DO NOT pierce the meat while searing to avoid losing juices.

After searing, transfer the steak to a cutting board and loosely tent with foil. Rest for 8–10 minutes — resting lets muscle fibers relax and juices redistribute so the sliced steak remains juicy instead of leaking out on the plate.

While the steak rests, reheat and crisp the halved potatoes: in the same skillet over medium heat, add the butter and minced garlic (if using) and toss the halved potatoes cut-side down until golden, 3–4 minutes. Season with salt and pepper. Crisping adds texture and caramelized flavor that contrasts with the tender interior.

Toss the blanched green beans lightly in a tablespoon of the vinaigrette to season them (they’ll pick up flavor without needing heavy oil). Dress the baby gem or mixed lettuces with a small drizzle of vinaigrette (about 1–2 tsp per serving); you want the lettuce lightly coated so it doesn’t become soggy.

Slice the rested flank steak thinly across the grain — slicing against the grain shortens muscle fibers and makes each bite more tender. Aim for 3–5 mm slices.

Assemble the plates family-style or individually: arrange a bed of dressed lettuce, place warm halved potatoes, a small pile of dressed green beans, fan the sliced steak, and top with a halved soft‑boiled egg (yolk facing up). Scatter a few capers and chopped anchovy (if using) for salty, umami notes and finish with a final drizzle of vinaigrette and a grind of black pepper.

Serve immediately so the warm steak and potatoes meet the cool, crisp lettuce and the yolk remains jammy. Encourage guests to break the yolk so it enriches the vinaigrette and unites the plate.