Using a small, sharp pairing knife, cut the very center of the onion out (about ½-inch in diameter).
Turn the onion upside down, trim and long roots (but don't remove the root ball, this will hold the onion together), and then make about 8 slices down the length of the onion.
Flip over, and gently separate the "petals" with your fingers. Place in a small bowl and pour the buttermilk over the top. Refrigerate for 30 minutes to overnight.
Remove the onion and let it drain while you prepare the wet and dry batters. In the buttermilk, add the eg and hot sauce, and whisk to combine.
Place the flour, salt, garlic powder, black pepper, and cayenne pepper in another bowl, and whisk to combine.
Dip the onion in the wet batter, making sure it coats as many of the "petals" as possible. Then place it in the bowl of the dry batter and do your best to coat every petal surface with the dry batter.
Place in the air fryer and spray with the avocado oil. Cook at 350°F for 20 minutes, spray again and rotate if necessary, then cook for the remaining 15 minutes.
While the onion cooks, add the dipping sauce ingredients to a bowl and mix together until smooth.
Serve the blooming onion with the dipping sauce and enjoy!