Drain the canned tuna thoroughly and place it in a medium mixing bowl.
Use a fork to flake the tuna into small, bite-sized pieces.
Finely mince the red onion and dice the celery stalk into small cubes.
Add the minced red onion, diced celery, capers, and chopped fresh parsley to the bowl with the tuna.
In a separate small bowl, whisk together the extra virgin olive oil, lemon juice, and Dijon mustard until emulsified.
Pour the dressing over the tuna mixture and fold gently with a spatula to combine all ingredients.
Season the salad with sea salt and black pepper to taste.
Gently fold in the diced avocado, being careful not to mash it too much.
Arrange the mixed baby greens on a large plate or in a bowl.
Scoop the Mediterranean tuna salad onto the center of the greens and serve immediately.