Whisk together avocado mayonnaise, Dijon mustard, pepperoncini brine, lemon juice, lemon zest, granulated garlic, salt, and pepper in a small bowl until smooth.
Fold in red onion, celery, pepperoncini, scallions, dill, and basil until everything is evenly coated.
Gently fold in tuna, keeping some larger pieces intact for texture.
Taste and adjust seasoning: add more brine if you want it punchier, more lemon if you want it brighter, and a pinch of salt to make everything pop.
Let it sit for 10 to 15 minutes if you have time so the flavors meld together. Serve topped with feta, pickled onion, and/or a spritz of lemon juice.