In a large bowl, combine the drained tuna, diced apple, chopped celery, and finely chopped red onion.
In a small bowl, whisk together the Greek yogurt, mayonnaise, Dijon mustard, lemon juice, salt, and pepper until smooth.
Pour the dressing over the tuna mixture and toss until everything is evenly coated.
Serve the salad on a bed of lettuce leaves.
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