Cut the top off the onion and peel it. Place the onion on a flat surface, root-side up. Cut down through the onion from top to bottom, stopping about 0.5 inch from the root. Repeat this to make four equal sections.
Make additional cuts between the initial cuts, creating a petal-like effect. You should have about 16-20 petals.
In a bowl, combine flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, salt, and pepper. In another bowl, pour the buttermilk.
Carefully dip each onion petal into the buttermilk, then dredge in the flour mixture, making sure to coat each petal thoroughly.
Heat vegetable oil in a large pot or deep fryer to 350°F (175°C).
Gently place the onion into the hot oil and fry for 3-4 minutes, or until golden brown and crispy.
Remove the onion from the oil and place on a wire rack to drain. Serve immediately.
Notes
Serve with your favorite dipping sauce, such as ranch dressing or a spicy mayo.