Flat Belly Foods: 400-Calorie Lunch Recipes: Greek Salad with Tuna In a large bowl, whisk together 2 teaspoons olive oil, 2 teaspoons red wine vinegar, 1/2 teaspoon dried oregano, and salt and pepper to taste. Toss with 3 cups chopped romaine, 2 1/2 ounces water-packed tuna, 1/2 cup diced cucumber, 1/2 cup diced tomato, 1/2 cup cooked whole-grain couscous, 1 tablespoon crumbled feta, and 4 chopped Kalamata olives.