Drain the tuna thoroughly and place it in a large mixing bowl. Flake it apart using a fork.
Add mayonnaise, lemon juice, and Dijon mustard. Mix until the tuna is evenly coated and creamy.
Add finely chopped red onion, diced celery, and halved cherry tomatoes. Stir gently to combine.
Season with salt and black pepper. Adjust the taste to your preference.
Arrange lettuce leaves in a serving bowl and spoon the tuna mixture on top.
Top with avocado slices and serve immediately. You can also chill it for 15 minutes before serving for extra freshness.
Notes
Serve this tuna salad with crackers, on toasted bread, or inside wraps. For extra flavor, add herbs like dill or parsley, or replace mayo with Greek yogurt for a lighter version.