In a medium bowl, combine the tuna, celery, onion, mayonnaise, parsley, pickle relish, salt, and pepper. Taste and adjust the seasonings if necessary.
Top one bread/toast slice with some of the tuna mixture. Spread mixture evenly. Add lettuce, tomato, etc., if desired. Top with another bread/toast slice. Cut in half if desired.
Serve immediately.
Notes
I'm often asked about the addition of eggs. Feel free to add a couple of chopped hard-boiled eggs to the tuna mixture if you like it that way! Depending on the consistency you like, you may need to add a tad more mayo.
Not a fan of sweet pickle relish? Try dill relish - or omit it entirely.
I love adding fresh herbs to my tuna salad when I have fresh herbs on hand. Thyme, basil, dill, and parsley are all great.
Want to change it up a bit? Try adding some Cajun seasoning ... it's awesome!
Refrigerate leftovers in an airtight container for up to 3-5 days.