Drain and flake the canned tuna into a large mixing bowl.
Add finely diced celery to the bowl.
In a separate small bowl, whisk together Greek yogurt, lemon juice, Dijon mustard, olive oil, minced garlic, chopped onion, and chopped parsley.
Pour the dressing over the tuna and celery mixture.
Season with salt and black pepper. Stir until well combined.
Transfer to a serving dish or store in an airtight container in the refrigerator until ready to serve.