Prepare the Tuna: Sear the fresh tuna steaks on both sides until medium-rare. Let them cool, then slice thinly. If using canned tuna, simply drain.
Boil the Eggs: Hard-boil the eggs, cool them in ice water, then peel and halve them.
Cook the Vegetables: Boil the potatoes until tender. Blanch the green beans in boiling water, then cool in ice water to retain their color.
Assemble the Salad: On a large platter, arrange the mixed greens. Top with tuna slices, halved cherry tomatoes, hard-boiled eggs, Niçoise olives, red onion slices, green beans, potato quarters, capers, and anchovy fillets (if using).
Make the Dressing: In a small bowl, whisk together olive oil, red wine vinegar, Dijon mustard, and fresh herbs. Season with salt and pepper. Drizzle the dressing over the salad.
Serve: Serve the salad immediately after dressing to keep the greens crisp.