In a large bowl, combine the drained and rinsed white beans, drained tuna, diced cucumber, and thinly sliced red onion.
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, and minced garlic.
Pour the dressing over the bean and tuna mixture and toss to coat.
Season with salt and pepper to taste.
Serve chilled or at room temperature.
