Instructions

1. Place the potatoes, unpeeled, in a pot and cover with cold water, making sure the water rises 1-1.5 inches (2-3 cm) above the potatoes. Add a little salt and bring to a boil. Once the water boils, reduce the heat and simmer until the potatoes are tender. When cooked, cool them quickly in very cold water, then peel.
   
2. Cut the potatoes into large chunks. Peel the cucumber, remove the seeds, and slice it into thick half-moons. Cut the tomatoes into large pieces. Clean and slice the onion thinly.
   
3. In a large bowl, combine the potatoes, cucumber, tomatoes, and onion. If desired, add the tuna. Season with olive oil, salt, and oregano, and toss gently to mix.
   
4. Let the salad rest in the refrigerator for at least 30 minutes.