This salad is so versatile, it can be made with chicken instead of tuna and any variety of leafy green besides arugula. The beans can be of your choice as well since beans provide lots of fiber.
Place beans into serving dish. Pour the tuna evenly over the beans. Place the arugula on top of the tuna.
In a separate, small bowl, prepare the dressing by whisking together the oil, lemon juice, salt and parsley until well combined.
Pour dressing over the tuna and beans. Sprinkle onions over the bean salad and mix well. Serve.