Drain the canned tuna thoroughly and place it in a mixing bowl.
Add the mayonnaise and Dijon mustard to the bowl with the tuna.
Finely chop the celery and red onion, then add them to the mixture.
Slice the black olives and add them to the bowl.
Chop the fresh parsley and mix it in.
Season the mixture with salt and pepper to taste.
Gently stir all the ingredients until well combined.
Prepare a plate with mixed greens as a base.
Spoon the tuna mixture over the greens.
Garnish with cherry tomatoes and extra parsley if desired.