In a bowl combine tuna, mayonnaise, red onion, parsley and lemon juice. Add salt and pepper and mix well.
Slice the avocado in half lengthwise and remove the pit. Scoop out avocado to create bowls, leaving a small border. Dice scooped out the avocado and fold into tuna mixture.
Fill avocado bowls with tuna mixture and top with tomato quarters. Serve & enjoy!