Prep and drain the tuna: Open both 120 g cans of tuna, drain into a small bowl and reserve 1 tablespoon of the oil from the cans. Flake the tuna gently with a fork and set aside. Why: reserving a bit of the tuna oil amplifies the tuna's flavor in the dressing; gentle flaking preserves pleasant flakes rather than a paste.

Prep vegetables: Rinse the baby potatoes and trim the ends off the green beans. Cut any larger potatoes in half so all pieces are similar size. Why: even sizing ensures uniform cooking; rinsing removes dirt and starch from the potato skins so they cook cleanly.

Boil the potatoes: Place the potatoes in a medium saucepan and cover with cold water by 1–2 cm (this helps them heat evenly). Add 1 tsp kosher salt and bring to a simmer over medium-high heat. Cook until just tender when pierced with a small knife or fork (about 12–15 minutes for 300 g of baby potatoes). Why: starting in cold water and bringing to a simmer cooks the potatoes evenly from center to edge, avoiding a mealy outside and raw center.

Add the green beans: When the potatoes have about 4 minutes left, add the trimmed green beans to the same pot. This finishes them tender-crisp in the same flavored water and saves time and pans. Cook for 3–4 minutes until bright green and just tender. Why: cooking the beans briefly keeps them snappy and colorful — overcooked beans become soft and lose vibrancy.

Shock and dry: Drain the potatoes and green beans into a colander and immediately plunge them into a bowl of ice water to stop the cooking and set the color (leave for 1–2 minutes). Drain thoroughly and pat dry with a clean kitchen towel. Why: an ice bath preserves that vivid green color and prevents carry-over cooking so textures remain perfect.

Make the dressing (if using the provided 3 tbsp dressings, you can whisk in reserved tuna oil and minced shallot): In a small bowl whisk together the reserved 1 tbsp tuna oil, 3 tbsp olive oil & lemon dressing (or 2 tbsp extra-virgin olive oil + 1 tbsp fresh lemon juice if making from scratch), 1 tsp Dijon, the minced shallot, minced garlic, a pinch of salt and a generous grind of black pepper. Taste and adjust acidity with the remaining 1/2 lemon if needed. Why: emulsifying mustard into the oil and lemon binds the dressing so it clings to potatoes and beans rather than running off.

Warm the potatoes briefly with dressing: While still slightly warm, slice the potatoes in half (or into 1–1.5 cm slices). Place them in a mixing bowl and toss with about half the dressing so the warm potato absorbs flavor. Why: warm potatoes soak up vinaigrette better than cold ones, giving more cohesive flavor.

Prepare the greens and assemble base: In a large serving bowl, toss the mixed greens lightly with a small drizzle (about 1 tsp) of the dressing — you want them lightly coated, not saturated. Arrange dressed greens on the serving platter or divide between two plates. Why: keeping greens only lightly dressed prevents them from going soggy when other components are added.

Arrange the salad: On top of the greens arrange the warm dressed potatoes, green beans, flaked tuna (in gentle mounds so the flakes remain intact), olives, and capers. Drizzle remaining dressing over the whole salad, then finish with a squeeze of lemon and a grind of pepper. Why: assembling warm and cold elements strategically balances temperature and allows the dressing to meld flavors while keeping textures distinct.

Final finish: Scatter chopped parsley for freshness and optional lemon zest for brightness. Taste and adjust salt/pepper. Serve immediately. Why: the herb and lemon zest give a final aromatic lift and make the dish feel finished and restaurant-ready.