Bring a medium pot of water to boil and season generously with salt. Meanwhile, trim the ends of 350g green beans, thinly slice 1 red onion and make a batch of Italian dressing. When the water boils, drop the green beans and cut the heat. Let cook for 2-3 minutes, or until bright green. Meanwhile, prepare an ice bath. When the beans are green, transfer to the ice water, along with sliced red onion. After 1 minute, remove and transfer to a salad spinner or clean dish towel to dry.
To a small bowl, add red onion slices, 1 teaspoon sumac and the juice of ½ lemon, toss and set aside while you make the salad.
To a shallow serving bowl, add cooked green beans, 1 cup pitted Kalamata olives and ¼ cup capers. Drain the oil from 3 cans of tuna, then add to the bowl and cover everything in Italian dressing. Toss to evenly coat, then finish with sumac red onions and a drizzle of olive oil.