There is a good blooming onion batter recipe on FoodNetwork.com. To use the batter, make sure to get a 1 pound Vidalia onion and cut it before dipping.
To make the batter, whisk together 2 1/2 cups of flour, 1 teaspoon of cayenne pepper, 2 tablespoons of paprika, 1/2 teaspoon of black pepper and 1/2 teaspoon each of thyme, oregano and cumin. In a small deep bowl, mix 2 eggs, 1 cup of whole milk and 1 cup of water. Place the cut onion in a third bowl, cut-side up, and pour the flour mixture over the top. Cover the bowl, and shake the onion back and forth to ensure the flour is distributed throughout the onion. Lift the onion by the core, and pat the excess off.
Use a slotted spoon to submerge the onion in the liquid mixture. Allow the excess to drip off, and then put the onion in the flour again before putting it in the refrigerator to cool. Heat a gallon of corn or soy oil in a deep pot until it rises to 400 degrees. Take the onion from the refrigerator, and using a skimmer, lower the onion into the oil, cut-side down. Lower the heat about 50 degrees, and fry for about three minutes before turning the onion over. Cook for three additional minutes, drain, and serve hot.