In a large mixing bowl, combine the drained tuna, Greek yogurt, lemon juice, and Dijon mustard.
Add the diced celery, chopped red onion, and cucumber to the bowl. Mix everything together until well combined.
Drizzle with olive oil and season with salt and black pepper to taste.
Garnish with fresh parsley and serve immediately, or refrigerate for 15 minutes to allow the flavors to meld.
Notes
This salad can be served on a bed of greens, in a whole grain wrap, or as a filling for a sandwich. You can also add avocado or hard-boiled eggs for extra richness and protein.