Craft the Vinaigrette: In a small bowl or glass jar, combine all vinaigrette ingredients: extra virgin olive oil, lemon juice, fresh oregano, minced garlic, Dijon mustard, and lemon zest. Whisk vigorously or seal the jar and shake until well emulsified. Season generously with black pepper. Set aside to allow the flavors to meld.
Cook the Potatoes: Fill a large pot with water and bring to a rolling boil. Add the scrubbed fingerling potatoes and cook for 15-20 minutes, or until fork-tender. Using a slotted spoon, carefully transfer the potatoes to a cutting board to cool slightly. Keep the water boiling.
Cook the Eggs & Beans: Gently lower the eggs into the same pot of boiling water and set a timer for 7 minutes for perfect jammy yolks (or 10-12 minutes for fully hard-boiled). During the last 2 minutes of the eggs' cooking time, add the trimmed green beans to the pot to blanch.
Chill the Eggs & Beans: Prepare a large bowl with ice and water. When the timer goes off, use the slotted spoon to immediately transfer both the eggs and the green beans into the ice water bath. This stops the cooking process and preserves the vibrant green color of the beans. Let them chill for 5 minutes. Drain the green beans completely. Carefully peel the cooled eggs and slice them in half lengthwise.
Dress the Potatoes: While the eggs and beans cool, slice the slightly cooled potatoes in half or into thick rounds. Place them in a medium bowl and drizzle with 2-3 tablespoons of the prepared vinaigrette. Toss gently to coat. Allowing the warm potatoes to absorb the dressing is key to a more flavorful salad.
Assemble the Platter: Spread the arugula and radicchio mix across the surface of a large serving platter to create a lush base. Artfully arrange the ingredients in distinct sections on top of the greens: the dressed potatoes, crisp green beans, halved cherry tomatoes, sliced cucumber, red onion, radishes, rinsed chickpeas, and low-sodium olives.
Finish and Serve: Flake the drained tuna into large chunks and nestle it among the other ingredients. Place the halved jammy eggs strategically around the platter. Just before serving, drizzle the remaining Zesty Oregano-Lemon Vinaigrette over the entire assembly. Serve immediately.