Flake tuna into a small bowl with a fork, salt and pepper. Put scallions and celery into a mini-food processor, pulse until fine, add to tuna. Fold in mayonnaise, mix well.
Meanwhile, preheat your griddle. Next assemble sandwiches. Spread tuna mixture evenly over 2 slices rye bread. Top with 1-1/2 slices Swiss cheese and remaining rye bread. Turn over, butter outside of tuna side. Place, butter side down on griddle, butter top.
When toasty brown, flip sandwich over and toast remaining side. Remove to sandwich plate, slice and serve.