Using a paring knife, carefully extract a ½-inch diameter core from the onion’s top center. Flip the onion and trim the long roots without removing the root base, which keeps the structure intact. Slice the onion vertically into approximately eight sections.
Turn the onion upright and gently tease apart the segments to form "petals." Submerge the onion in a bowl filled with buttermilk, then refrigerate for at least 30 minutes or overnight.
Remove the onion from the buttermilk, allowing it to drain. In the remaining buttermilk, blend in the egg and hot sauce to form the wet batter. In a separate bowl, mix the flour, salt, garlic powder, black pepper, and cayenne pepper to prepare the dry coating.
Thoroughly immerse the onion in the wet mixture, ensuring all petals are covered. Transfer to the dry mixture, meticulously coating each surface.
Position the onion in the air fryer basket and apply a light mist of avocado oil spray. Air fry at 350°F for 20 minutes. Afterward, spray again, rotate if needed, and continue cooking for an additional 15 minutes.
While the onion cooks, combine the sour cream, mayonnaise, dill, garlic powder, cayenne, salt, and black pepper in a bowl until well blended.
Serve immediately alongside the prepared dipping sauce.