Two 6-ounce cans white meat tuna packed in water, drained
2 tablespoons minced celery
2 tablespoons minced red onion, soaked in cold water for 5 minutes and drained
1 teaspoon minced flat-leaf parsley
1/3 cup prepared mayonnaise
1 tablespoon whole-grain mustard
Freshly ground black pepper
Freshly squeezed lemon juice (optional)