Put the red pepper into a saucepan, add water to cover, and place over high heat. Bring to a boil and cook uncovered for 1 minute. Drain and set aside. Flake the tuna into a large bowl. Add the beans, scallions, red pepper, and basil. Stir to blend, then sprinkle with lemon zest.
Using a pestle and small mortar, work the salt and garlic into a paste. Add the lemon juice and stir with the pestle until the salt is dissolved. Transfer the paste to a small bowl. Gradually add the oil, whisking constantly. Season with black pepper and whisk to blend.
Pour the dressing over the bean mixture and toss to coat. Serve on a bed of lettuce and top with black olives.