01 - Peel the onions and trim the root ends, ensuring each onion can stand upright. Make four evenly spaced vertical cuts into each onion from top down, leaving the base intact. Rotate and create four additional cuts between the first set to form even petals. Gently separate the petals by hand.
02 - Place cut onions in a bowl of ice water and let soak for 10 minutes to help open the petals. Remove the onions, drain thoroughly, and pat dry with paper towels.
03 - In a mixing bowl, combine all-purpose flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and black pepper. In a separate bowl, whisk together eggs and whole milk until well blended.
04 - Dredge each onion in the flour mixture, shake off excess, then dip into the egg mixture ensuring even coverage. Return the onions to the flour mixture and coat thoroughly, pressing the petals to help the batter adhere.
05 - Pour vegetable oil into a deep fryer or heavy-bottomed pot to a depth of at least 2 inches. Heat oil to 350°F (175°C).
06 - Working in batches, carefully lower the battered onions into the hot oil, cut side down. Fry for 2 to 3 minutes, flip each onion, and continue frying another 2 to 3 minutes until uniformly golden and crisp. Transfer to paper towels to drain.
07 - In a small bowl, mix mayonnaise, ketchup, prepared horseradish, smoked paprika, garlic powder, salt, and pepper until smooth and well combined.
08 - Serve the mini bloomin' onions hot, accompanied by dipping sauce.