1. Cook the eggs for 10 minutes in boiling water. Once cooked, remove them and rinse them under cold water. Set aside. Pour a little water into a pot and bring it to a boil. Place the green beans in the steamer basket of the pot, cover, and let them heat for about 10 minutes.
2. Meanwhile, drain the tuna and crumble it. Wash the lettuce and spin it dry. Wash the tomatoes and cut them into quarters. Take the hard-boiled eggs and peel them. Cut them into quarters. In a salad bowl, mix the lettuce, green beans, tomatoes, and hard-boiled eggs. Add a few black olives.
3. Prepare the light vinaigrette. In a small bowl, mix the salt and vinegar. Add the mustard, water, and oil, whisking with a fork, then season with pepper to taste. For a creamier dressing, you can add 1 to 2 teaspoons of 0% fat Greek yogurt. Pour the dressing over the salad and serve.