Drain tuna thoroughly and transfer to a mixing bowl. Break into flakes with a fork.

Add mayonnaise, Dijon mustard, and lemon juice. Mix gently until combined.

Fold in chopped celery, red onion, and optional parsley.

Add sweet relish or pickles if using and mix lightly.

Season with salt and black pepper to taste.

Cover and refrigerate for about 20 minutes to allow flavors to develop.

Stir gently before serving and adjust seasoning if needed.

Serve in sandwiches, wraps, or over greens.