Drain the canned tuna thoroughly and place it in a medium mixing bowl, flaking it into bite-sized pieces with a fork.
Finely dice the red onion and English cucumber, and slice the cherry tomatoes into halves.
Add the diced onion, cucumber, tomatoes, capers, and chopped fresh parsley to the bowl with the tuna.
In a small separate jar or bowl, whisk together the extra virgin olive oil, lemon juice, sea salt, and black pepper until well combined.
Pour the dressing over the tuna and vegetable mixture, tossing gently to ensure everything is evenly coated.
Arrange the fresh arugula on a large serving plate or in a bowl and top it with the Mediterranean tuna salad.
Slice the avocado and place it on top of the salad just before serving for a creamy finish.