In a small mixing bowl break up the tuna with a fork. Toss with the celery, onion and parsley. Add the mayonnaise, mustard and season with pepper, to taste.
Stir to combine. Add lemon juice, to taste, if using.
Notes
Variation: Substitute 1 teaspoon Dijon mustard for the whole-grain mustard and add 2 tablespoons sweet pickle relish.