01 - Peel onions and trim the root end just enough so the onions stand upright. Make four evenly spaced vertical cuts from the top downward, leaving the base intact. Rotate and make four more cuts between the first to create petals. Gently separate the petals with your fingers.
02 - Place prepared onions in a bowl of ice water for 10 minutes to encourage blooming. Drain thoroughly and pat each onion dry.
03 - Whisk together all-purpose flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper in a large bowl.
04 - In a separate bowl, beat eggs and whole milk until well combined.
05 - Dredge each prepared onion in the flour mixture, tapping off excess. Dip in the egg mixture, then dredge again in the flour mixture. Press gently to ensure the coating adheres to all petals.
06 - Pour vegetable oil into a deep fryer or heavy-bottomed pot to a depth suitable for submerging onions. Heat to 350°F (175°C).
07 - Working in batches, carefully lower onions cut side down into the hot oil. Fry for 2 to 3 minutes, flip and fry an additional 2 to 3 minutes, until golden and crispy. Remove with a slotted spoon or spider and drain on paper towels.
08 - In a small bowl, mix together mayonnaise, ketchup, horseradish, smoked paprika, garlic powder, salt, and pepper until smooth and creamy.
09 - Arrange hot crispy bloomin' onions on a plate and serve immediately with the prepared dipping sauce.