Step 1In a medium bowl, break up the tuna with a fork and stir in the celery, cucumber and white onion.
Step 2In a separate small bowl, stir together the mayonnaise, lemon and orange zest, green onion, parsley and salt. Fold the mayonnaise into the tuna until it is evenly mixed. Chill until ready to serve, at least 1 hour.
Step 3Just before serving, divide the tuna mixture evenly among 4 chilled plates lined with a lettuce leaf. Garnish each with 3 grapefruit slices and a pita half.