Preheat oven to 400 degrees. Use a large knife to cut off the top about 1/4-inch of the onion until a few of the inside layers are exposed. Be careful to cut the top side, not the root side. Peel the outermost layer of the onion down to the root, leaving the root as is.
Lay the onion cut side down on the cutting board. Then use a knife to section the onion into 16ths, beginning with your knife 1/8-inch away from the root and cutting straight down. You can cut more sections if you have an especially large onion. Once all cuts are complete, turn the onion over and set on a piece of parchment paper on a baking sheet. The onion will fall open, but you' ll also want to use your fingers to separate the onion petals and open it completely.
In a small bowl, combine panko, paprika, Cajun seasoning, and salt. Whisk the egg whites together in a separate bowl.
To coat the onion, I started in sections. I would coat each onion petal with egg whites and then sprinkle liberally with the panko mixture. I worked from the bottom to the top in each section, and then turned the onion to work on the next section. I brushed the entire onion with one more layer of egg white and sprinkled the mixture over the entire onion once done with the sections.
Bake covered with aluminum foil for 5 minutes, then uncovered for an additional 10-15 minutes, or until the onion is tender and the edges are browned.
This recipe was adapted from one of my favorite blogs, Gimme Some Oven. She also includes an awesome sauce that tastes great with this onion!