01 - Peel the onions and trim the base slightly so they stand upright. With a sharp knife, make four evenly spaced vertical cuts in each onion, slicing almost to the base but not through. Rotate and make four additional cuts, forming petals. Gently separate the onion petals with your fingers.
02 - Submerge the prepared onions in ice water for 10 minutes to encourage blooming. Remove, drain fully, and pat onions dry with paper towels.
03 - In a large mixing bowl, whisk together all-purpose flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper until evenly combined.
04 - In a separate mixing bowl, beat eggs and whole milk until fully blended.
05 - Dredge each dried onion thoroughly in the seasoned flour mixture, shaking off any excess. Dip the coated onion into the egg-milk mixture, then dredge again in the flour blend, pressing the petals gently to ensure an even coating.
06 - Pour vegetable oil into a deep fryer or heavy-bottomed pot to a depth of at least 2 inches and heat to 350°F.
07 - Working in batches, carefully lower onions cut side down into the hot oil. Fry for 2 to 3 minutes, turn, and continue frying another 2 to 3 minutes until onions are golden brown and crisp. Transfer to paper towels to drain excess oil.
08 - In a small bowl, thoroughly mix mayonnaise, ketchup, prepared horseradish, smoked paprika, garlic powder, and season with salt and black pepper until smooth.
09 - Arrange crispy bloomin' onions on a serving platter and accompany with the prepared dipping sauce. Serve while hot.