Tuna sauce is a classic Piedmontese accompaniment to cold roasted meats, most notably in Vitello Tonnato (veal with tuna sauce). Here we've capitalized upon the same flavor components - rich tuna, briny capers, refreshing parsley and lemon - to make a creamy sauce that enlivens fresh green beans. This is a dish meant to be served cold; it's a great addition to any summer party or picnic. Be sure to use tuna packed in olive oil for the best flavor and texture. Imported tonno (tuna) is worth seeking out, or look for Dave's Gourmet Albacore, a sustainably hook-and-line-caught product with no additives.
Bring a large pot of water to a boil. Add the 1 tablespoon salt and the beans, and cook over high heat, uncovered until the beans are crisp-tender, 4 to 5 minutes. Drain immediately and transfer the beans to a bowl of ice water to stop the cooking process. When the beans are cold, drain again and dry them with a clean towel. Transfer the beans to a platter and set aside.
Meanwhile, place the tuna, olive oil, mayonnaise, lemon zest and juice, 1 tablespoon of the capers, and the parsley in a blender. Process until very smooth, scraping down the sides of the container several times. Add a bit of water if needed to thin the sauce to the consistency of heavy cream. Season to taste with salt and pepper.
To serve, drizzle the sauce over the beans. Garnish with the reserved capers and wedges of lemon.