01 - Peel onions and trim just enough of the root so onions stand flat. Create four evenly spaced vertical cuts downward on each onion, leaving the base intact. Make four more cuts between the previous ones for a total of eight sections. Gently separate petals with your fingers.
02 - Place onions in a bowl of ice water for 10 minutes to help them open. Drain thoroughly and pat dry with paper towels.
03 - In a mixing bowl, combine flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper. Whisk until evenly blended.
04 - In a separate bowl, beat eggs and whole milk together until smooth.
05 - Dredge each onion in the flour mixture and shake off the excess. Submerge in the egg mixture, then coat again in the flour mixture, pressing gently to ensure petals are thoroughly covered.
06 - Heat vegetable oil in a deep fryer or heavy-bottomed pot to 350°F (175°C).
07 - Working in batches, carefully lower onions cut side down into hot oil. Fry for 2 to 3 minutes, flip, and continue frying another 2 to 3 minutes until they are golden and crisp. Remove with a slotted spoon and transfer to paper towels to drain.
08 - For the dipping sauce, mix mayonnaise, ketchup, horseradish, smoked paprika, garlic powder, salt, and black pepper in a bowl until smooth.
09 - Serve mini bloomin onions hot, accompanied by the dipping sauce.