Preheat the air fryer to 375°F (190°C). Trim the top and root end of the onion (leaving the root intact), peel, and cut into 12–16 wedges from the top down without cutting through the base. Gently spread the petals open. Soak in cold water 30 minutes if time allows.
Whisk the flour with onion powder, garlic powder, paprika, salt, and cayenne in one bowl. Whisk eggs and buttermilk in another.
Dip the onion in the egg mixture, then dredge in the seasoned flour. Repeat both dips for full coverage.
Spray all over with cooking spray. Place flat in the basket and air fry 15–20 minutes, flipping halfway, until deep golden and crisp.
While it cooks, whisk all sauce ingredients together and chill.
Rest the onion a few minutes, then serve hot with the sauce.