In a bowl, whisk together avocado mayo, Dijon mustard, lemon zest, lemon juice, peperoncini brine, granulated garlic, salt, and pepper until smooth
Fold in the red onion, celery, peperoncini, scallions, dill, and basil so everything is evenly coated
Gently fold in the tuna, keeping some larger pieces intact for texture
Taste and adjust seasoning by adding more brine for punchiness, more lemon for brightness, and salt to enhance flavors
Let it sit for 10–15 minutes if you have time so the flavors meld together
Finish with crumbled feta, lemon juice, and pickled onion
