In a small mixing bowl, whisk together the extra-virgin olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and black pepper until emulsified.
In a large bowl, combine the drained cannellini beans, cherry tomatoes, sliced red onion, and capers.
Gently fold the flaked tuna and olives into the bean mixture, taking care not to over-mix and break the tuna chunks too small.
Drizzle the vinaigrette over the salad and toss lightly to coat.
Incorporate the fresh chopped parsley.
Arrange a bed of arugula on a large serving platter or individual plates.
Spoon the tuna and bean mixture over the greens and serve at room temperature.