Combine the drained tuna, onion, pickle relish, and mayonnaise in a medium bowl. Gently mix the ingredients.
Use a small teaspoon to taste the mixture. Add salt as needed. Mix well.
Arrange the cheese on top of one slice of bread. Top with 1/3 of the tuna mixture. Place another slice of cheese on top of the tuna mixture, and then top with the second bread slice. Do this step until all 3 sandwiches are assembled.
Heat a skillet. Put one tuna sandwich on the heated skillet and let one side cook for 1 to 3 minutes or until the cheese starts to melt. Flip the sandwich to toast the other side.
Remove the sandwich from the skillet. Slice in half.
Serve. Share and enjoy!