Prepare the onion: Slice off the top 1/2 inch of the onion. Stand the onion cut-side down and make a cut from top to bottom, about 3/4 inch from the root, stopping about 1/2 inch from the bottom. Repeat this cut around the onion, spacing cuts about 3/4 inch apart. Gently spread the onion petals open.
Prepare the dredge: In a shallow bowl, whisk together flour, paprika, salt, pepper, garlic powder, onion powder, oregano, thyme, and cayenne pepper (if using).
Prepare the egg wash: In another shallow bowl, whisk together milk and eggs.
Bread the onion: Dip the entire onion into the flour mixture, ensuring it's well coated and pushed down into the petals. Shake off excess.
Dip the onion into the egg wash, ensuring all parts are coated. Let excess drip off.
Coat with breadcrumbs: Sprinkle the breadcrumbs evenly over the onion, gently pressing them into the petals. You can also carefully place the onion into the breadcrumbs, cupping it to coat.
Heat the oil: Heat vegetable oil in a deep fryer or large pot to 350°F (175°C).
Fry the onion: Carefully place the breaded onion into the hot oil, root-side down. Fry for about 3-4 minutes, or until golden brown and cooked through. Be careful not to overcrowd the fryer if making multiple.
Drain and serve: Remove the bloomin' onion with a slotted spoon or spider strainer and drain on a paper towel-lined plate. Serve immediately with your favorite dipping sauce.