- 1
Prepare the Onion: Slice off about 1/2 inch from the top (stem end) of the onion. Peel the outer layers. Place the cut top flat on a cutting board. Using a sharp knife, make a cut 1/2 inch from the root, straight down to the base, but do not cut through the root. Repeat to make 12-16 evenly spaced cuts around the onion, ensuring they all meet at the root but don't cut through it. Gently separate the "petals." Place the cut onion in a large bowl of ice water for at least 30 minutes to help it open up.
- 2
Make the Cajun Horseradish Sauce: In a small bowl, whisk together all sauce ingredients until smooth. Taste and adjust seasoning or horseradish level. Cover and refrigerate until serving.
- 3
Prepare the Dry Dredge: In a large bowl, whisk together 1 cup all-purpose flour, 1/4 cup cornstarch, 1/2 tbsp paprika, 1/2 tsp garlic powder, 1/2 tsp onion powder, 1/2 tsp salt, and 1/4 tsp black pepper.
- 4
Prepare the Wet Batter: In another large bowl, whisk together the remaining 1 cup all-purpose flour, 1/4 cup cornstarch, 1 tbsp baking powder, 1/2 tbsp paprika, 1/2 tsp garlic powder, 1/2 tsp onion powder, 1/2 tsp cayenne pepper, 1/2 tsp salt, and 1/4 tsp black pepper. In a separate bowl, whisk together the eggs, milk, and water (or beer). Pour the wet mixture into the dry ingredients and whisk until just combined (a few lumps are okay).
- 5
Heat the Oil: In a large, heavy-bottomed pot or Dutch oven, pour in the vegetable or peanut oil to a depth of about 3-4 inches (ensure there's enough room for the onion without overflowing). Heat the oil over medium-high heat to 350-375°F (175-190°C). Use a candy thermometer or deep-fry thermometer to monitor the temperature.
- 6
Batter the Onion: Remove the onion from the ice water and pat it thoroughly dry with paper towels, getting into all the crevices. Place the dried onion into the dry dredge mixture, pushing the petals down to ensure it's fully coated. Gently shake off any excess flour.
- 7
Transfer the floured onion to the wet batter. Use a spoon or ladle to pour the batter over and between all the petals, ensuring every part is coated.
- 8
Carefully transfer the wet-battered onion back into the dry dredge mixture. Gently press the flour mixture onto all surfaces, ensuring a thick, even coating. Shake off any excess.
- 9
Fry the Onion: Carefully lower the battered onion into the hot oil, cut-side down, using a large slotted spoon or spider. Fry for 8-10 minutes, or until golden brown and crispy. If necessary, carefully flip it halfway through frying to ensure even browning. The petals should be tender inside and crispy outside.
- 10
Drain and Serve: Carefully remove the fried onion from the oil and place it on a wire rack set over a baking sheet to drain excess oil. Season immediately with a pinch of salt. Place the onion on a serving plate and serve hot with the chilled Cajun Horseradish Sauce in the center.
