Prepare the onion: Start by cutting off a small portion of the onion's stem end to create a stable base. Peel the onion and cut off the top, leaving the root intact. Make vertical cuts from the top towards the root, making sure not to cut all the way through. The number of cuts will depend on the size of your onion, but aim for around 12 to 16 cuts.
Create the batter: In a bowl, combine the flour, paprika, garlic powder, salt, black pepper, and cayenne pepper (if using). Mix well to ensure that all the ingredients are evenly incorporated. Gradually add the milk to the dry mixture, whisking until a smooth batter is formed.
Coat the onion: Place the onion in a separate bowl and carefully pour the batter over it, making sure to evenly coat each petal. Gently press the batter into the cuts of the onion, ensuring that every crevice is filled. Let the onion sit in the batter for a few minutes to allow it to absorb the flavors.
Fry the onion: Heat oil in a deep fryer or a large pot to around 350°F (175°C). Carefully lower the battered onion into the hot oil, cut side down. Fry for about 3-4 minutes, then carefully flip the onion and fry for an additional 3-4 minutes until it is golden brown and crispy. Remove the onion from the oil and let it drain on a paper towel.
Serve and enjoy: Transfer the blooming onion to a serving plate and watch as it opens up, revealing its beautiful, crispy petals. Serve it hot with your favorite dipping sauces and enjoy the explosion of flavors with every bite.