Place the drained tuna in a large bowl. Use a fork to gently break it up into flakes.
Add the mayonnaise, zesty dressing (if using), and mustard. Stir until the tuna is evenly coated.
Add the minced onion, celery, parsley, garlic powder, and chopped pickles or relish (if using). Mix until well combined.
Season with salt and black pepper to taste. Adjust mayo or dressing if needed for your preferred creaminess.
Serve immediately on toasted bread with lettuce and tomato, or chill for 30 minutes for the best flavor.