Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Trim off the top of the onion, leaving the root end intact. Stand the onion cut-side down and make cuts from the top to about 0.5 inch from the root, dividing it into 12-16 wedges. Be careful not to cut all the way through.
Gently spread the wedges apart to form a blooming shape.
In a shallow bowl, whisk together the flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper (if using), salt, and black pepper.
In another shallow bowl, whisk together the beaten eggs and milk.
Dip the onion into the egg mixture, making sure to coat the inside of the wedges. Let any excess drip off.
Place the panko breadcrumbs in a third shallow bowl. Press the onion into the breadcrumbs, coating it thoroughly. Gently press the breadcrumbs into the crevices to ensure good coverage.
Place the breaded onion, cut-side down, on the prepared baking sheet.
Bake for 30-35 minutes, or until the onion is tender and golden brown. If it starts to brown too quickly, you can loosely tent it with foil.
Let it cool for a few minutes before serving. Serve with your favorite dipping sauce like ranch, bloomin' onion sauce, or a spicy aioli.
Notes
This baked version is significantly healthier than its deep-fried counterpart. Enjoy!