Cut off the stem end of each onion and peel. Place cut-side down on a cutting board. Using a sharp knife, make 4 to 6 cuts from top to bottom, stopping 1/2 inch from the root. Gently separate the layers with your fingers to create the “bloom.”
In one bowl, whisk together the eggs and milk.
In another bowl, mix the flour, cornstarch, garlic powder, paprika, onion powder, salt, and black pepper.
Dip each onion into the egg mixture, turning to coat and getting between the layers. Dredge in the flour mixture, pressing gently to coat every surface.
Repeat the process once more—dip back into egg, then into flour again.
Place each onion cut-side up in the air fryer basket. Spray generously with cooking spray or mist with oil. Air fry at 350°F (175°C) for 15–20 minutes, or until golden and crispy. Check near the end to avoid burning.