01 - Peel onions and trim root so each stands upright. Make 4 vertical cuts downward, without slicing through the base. Rotate and make 4 additional cuts between the previous cuts to create petals. Gently fan out the petals using your fingers.
02 - Place onions in ice water for 10 minutes to help petals open. Drain thoroughly and pat dry with paper towels.
03 - In a mixing bowl, whisk flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper until fully combined.
04 - In a separate bowl, beat the eggs with whole milk until smooth.
05 - Dredge each onion in the seasoned flour mixture and shake off excess. Dip into egg mixture, then coat again in flour mixture, pressing petals to ensure an even layer adheres.
06 - Bring vegetable oil to 350°F in a deep fryer or heavy-bottomed pot, ensuring there is enough oil to fully submerge the onions.
07 - Working in batches, carefully lower onions cut-side down into hot oil. Fry for 2–3 minutes, flip, and continue frying another 2–3 minutes until golden brown and crisp. Use a slotted spoon to transfer to paper towels for draining.
08 - Combine mayonnaise, ketchup, prepared horseradish, smoked paprika, garlic powder, salt, and pepper in a small bowl. Stir until smooth and well blended.
09 - Arrange warm bloomin’ onions on a serving platter. Serve immediately with prepared dipping sauce on the side.