Place the drained tuna into a small mixing bowl and flake it. Then add the mayonnaise, celery, scallions, Dijon, lemon juice, lemon zest, salt, and pepper.
Stir everything well until thoroughly mixed; taste and adjust salt if needed. Set aside.
Lightly toast some bread, and spread half of the tuna salad over a slice. Top it with some mixed greens.
Serve your sandwich with a side of chips and a pickle for the ultimate lunch plate!
Alternatively, you can snub the bread and enjoy the tuna salad by scooping it up with crackers. Or, make it a wrap!