Peel the onion and trim off the stem end. Cut off about 1/4 inch from the root end, but do not cut all the way through.
Make cuts from the top to about 1/2 inch from the root, dividing the onion into quarters. Then cut each quarter into thirds, so you have 12-16 evenly spaced cuts.
Gently spread the onion petals open to form a "flower" shape.
In a shallow dish, whisk together flour, cornstarch, paprika, cayenne pepper, salt, and black pepper.
In another shallow dish, whisk together eggs and milk.
Dip the onion into the egg mixture, making sure to coat all the petals. Let any excess drip off.
Dredge the onion in the flour mixture, gently pressing to coat evenly and ensuring flour gets into all the crevices.
Heat vegetable oil in a deep fryer or heavy-bottomed pot over medium-high heat to 350°F (175°C).
Carefully place the battered onion into the hot oil. Fry for 5-7 minutes, or until golden brown and crispy.
Remove the blooming onion from the oil with a slotted spoon and drain on paper towels.
Serve immediately with your favorite dipping sauce.
Notes
For a spicier kick, add more cayenne pepper. You can also serve with a variety of dipping sauces like ranch, honey mustard, or a spicy ketchup.