Besides packing a pop of protein and fiber, one serving of lentils delivers almost a full day’s worth of folate, a vitamin that promotes healthy cell growth.
Stir together water and lentils in a medium saucepan; bring to a boil over high. Cover and reduce heat to medium; simmer until lentils are tender, about 20 minutes. Drain and set aside.
Whisk together oil, lemon juice, salt, and pepper in a large bowl. Stir in parsley, mint, and onion to coat. Gently fold in beans, tomatoes, and lentils. Spoon mixture into a serving bowl; top with tuna.