Drain the tuna (if you have cats give them the juice or you're a monster).
In a small mixing bowl combine tuna, onion, mayonnaise, mustard, and season with pepper, to taste. If you have it, try it with lemon juice and parsley. Stir to combine.
Cut the ends off the celery and divide each stalk into 3. Spoon the tuna salad into the hollow of the stalk and serve.