01 - Trim root ends so onions stand upright. Make four evenly spaced vertical cuts without slicing through the base, then four more cuts between the first set. Gently loosen petals to open the onions.
02 - Immerse cut onions in ice water for 10 minutes to encourage blooming. Drain well and pat each onion dry with paper towels.
03 - In a large mixing bowl, whisk together flour, cornstarch, paprika, garlic powder, onion powder, cayenne, salt, and black pepper until well blended.
04 - In a separate bowl, beat eggs and whole milk until fully combined.
05 - Dredge each onion thoroughly in the seasoned flour mixture, shake off excess, then dip into egg mixture. Dredge again in flour mixture, ensuring all petals are evenly coated.
06 - Bring vegetable oil to 350°F in a deep fryer or heavy-bottomed pot.
07 - Working in batches, place onions cut side down into hot oil for 2–3 minutes. Turn and fry an additional 2–3 minutes until evenly golden and crisp. Use a slotted spoon to remove and drain on paper towels.
08 - In a small bowl, combine mayonnaise, ketchup, horseradish, smoked paprika, garlic powder, salt, and pepper. Mix until smooth and well blended.
09 - Arrange warm crispy onions on a platter and serve immediately with dipping sauce.