Prep the ingredients per the instructions above.
Flake the tuna with a fork, then combine it with the mayonnaise, onion, celery, relish, lemon juice, salt, and pepper.
Slather one side of each piece of bread with mayonnaise (like you would using butter), then place into a large skillet, mayonnaise side down.
Place a cheese slice onto the bread. Spread the tuna mixture over the cheese slices. Place a cheese slice on top.
Use the mayo to "butter " the remaining two slices, then place on top, mayonnaise side up and out.
Heat the pan to medium heat and cook until the bread is lightly browned on the bottom.
Flip the sandwiches carefully to the other side, then cook until the other side has browned.
Slice in half and serve with chips, pickle, and lemonade.