Drain the tuna. Place it in a bowl and use a fork to break apart any large clumps.
Finely chop the celery, red onion, and dill pickles.
For Classic Tuna Salad, use a fork to mix together the tuna and chopped vegetables with the Greek yogurt, mayo, lemon juice, Dijon mustard, and kosher salt. Add shredded Parmesan if desired (it really steps it up a notch!). For Tuna Salad with Egg, add chopped hard boiled egg. For Avocado Tuna Salad, use 1/2 chopped ripe avocado instead of the Greek yogurt and mayo, and skip the Parmesan.
Lightly toast the bread, if desired. Spoon the tuna salad onto the bread and serve.