Flake the tuna into a large mixing bowl. Add the mayo and stir until combined.

Add the pepper, onion powder, garlic powder, and celery seed and mix well with a fork to combine until all the seasonings are well incorporated. Add the chopped celery, bell pepper, and relish and mix again to combine. Taste the mixture and add salt and pepper if needed, and any extra seasonings to your liking.

To assemble the sandwiches before eating: spread about 1/3 of the tuna mixture onto one slice of bread. On the other slice, spread with more mayo if desired, I like to keep mine dry. Top with greens of your choice and enjoy!