Prepare the Onion: Cut off the top (stem end) of the onion.
Peel the onion, leaving the root end intact.
Make four evenly spaced vertical cuts into the onion, starting from the top and stopping before reaching the root, creating petals.
Create the Bloomin' Onion Batter: In a bowl, whisk together the flour, paprika, garlic powder, cayenne pepper, salt, and black pepper.
In another bowl, whisk together the eggs and buttermilk.
Coat the Onion: Dip the onion in the wet mixture, ensuring it gets between the petals.
Let the excess liquid drip off.
Roll the onion in the flour mixture, making sure to coat it thoroughly. Gently shake off any excess flour.
Double Dredge (Optional): Repeat the process of dipping the onion in the wet mixture and then in the flour mixture for an extra crispy coating.
Preheat the Oil: In a large, deep pot or a deep fryer, heat the vegetable oil to 350°F (180°C).
Fry the Bloomin' Onion: Carefully lower the coated onion into the hot oil, petals facing up.
Fry for about 3-4 minutes until the exterior is golden brown and crispy. Use a slotted spoon to turn the onion halfway through for even frying.
Drain Excess Oil: Remove the fried bloomin' onion from the oil and let it drain on a paper towel-lined plate.
Prepare the Dipping Sauce: In a small bowl, mix together mayonnaise, ketchup, horseradish, paprika, salt, black pepper, and cayenne pepper (if using). Adjust seasoning to taste.
Serve: Place the fried bloomin' onion on a serving plate.
Serve with the prepared dipping sauce in the center.
Enjoy: Pull apart the crispy petals, dip them in the sauce, and savor the deliciousness!