Preheat oven to 350 degrees. Spray 2-cup round baking dish with nonstick cooking spray.
Trim root end of onion so that it stands upright.
Peel and cut 1/2" from top.
With sharp knife, cut triangular slices to center of onion, slicing from top down and stopping 1/2" from bottom, working your way around onion to make several "petals."
Place upright in microwave-safe, covered dish and lightly coat in cooking spray.
Cover and microwave 2 minutes, until petals have begun to separate.
In small bowl, combine cornflake crumbs, peppers and salt.
In medium bowl, beat egg white until foamy.
Dip cooked onion into beaten white, coating petals thoroughly.
Place in prepared baking dish and sprinkle evenly with cornflake crumb mixture.
Bake until lightly browned and crisp, 10-12 minutes.
Combine sour cream, mayonnaise, horseradish and garlic.
Serve cooked onion immediately, with dip on the side