In a large mixing bowl, open the can of tuna and drain the liquid. Use a fork to flake the tuna into bite-sized pieces.

Incorporate the well-rinsed cannellini beans, finely chopped red onion, and freshly chopped parsley into the bowl with the tuna. Gently toss the ingredients together.

In a separate small bowl, whisk together the fresh lemon juice, olive oil, Dijon mustard, and dried oregano until smooth.

Pour the vinaigrette over the salad mixture and gently toss to coat all ingredients evenly.

Add salt and black pepper to taste, mixing thoroughly. Chill in the refrigerator for 15-20 minutes if desired before serving.