Heat your oven to 400 degrees F. Peel your onion, rinse it and pat it dry. Slice off the root and stem ends. Slice the onion into circles ½-inch thick (see photos). Separate the onion into rings by gently pushing through the center of each slice.
Place your corn starch in a quart- or gallon-sized plastic Ziploc bag. Stir your Parmesan coating ingredients together in a small mixing bowl. Place about a fourth of the Parmesan mixture into another Ziploc bag. (Later, when the cheese coating gets soggy and is almost gone, you will get a clean bag and add more of the Parmesan blend from your mixing bowl.)
Place two egg whites into a liquid measuring cup or a soup bowl. Use a fork to beat the egg whites until they are a little frothy.
Set up an assembly line from left to right with your onion rings, the corn starch, the cup or bowl of egg whites, the bag of Parmesan coating, and your baking sheet that has a wire mesh cooling rack placed on it. (See photos.) Have three forks ready: one for the corn starch, one for the egg whites and one for the cheese coating.
To dip, place an onion ring into the bag of corn starch, hold the bag closed, and shake it a bit until the onion ring is coated inside and out. Use a fork to loop inside the ring and lift it out, shaking off excess corn starch into the bag.
Drop the onion ring into the egg whites. With your egg white fork, scoop the egg whites over and inside the onion ring to coat it. Lift the ring out of the egg whites, letting any excess drip off.
Next, transfer the onion ring to the bag of Parmesan coating. Close the bag and shake it until the ring is coated inside and out. Use your Parmesan fork to lift out the onion ring and shake off excess cheese into the bag.
Place your coated onion ring onto the rack in your baking pan. Continue the process, adding two onion rings at a time (or a little more if they fit) to your corn starch bag. Remove one ring at a time from the corn starch and continue dipping that ring until it's ready to be placed on the rack. It helps to place rings of similar size next to each other on the rack, and you should be able to fit about 12 rings at a time on the rack.
Bake the onion rings for 11-14 minutes, or until they are crispy and reddish golden. Mine take 13 minutes, but it will depend on your oven. Don't let them burn.
While the first pan is baking, you can proceed to dip and coat the rest of your onion rings. You will need to use your other two fresh egg whites to replenish your cup or bowl of whites. You also will use a clean bag for more of the Parmesan mixture. (Change the bag as needed.)
Discard your bags with leftover corn starch or Parmesan mixture. If you have any leftover Parmesan blend from your mixing bowl, and it has not touched the raw egg whites, you can save it for another use.
Serve the onion rings plain or with melted butter for dipping, if desired, or marinara. (We love them plain or with the butter.)
Store leftovers in a tightly closed container in the refrigerator for up to five days.
I'm counting four onion rings as an appetizer serving.The onions themselves (inside the crisp coating) are crisp-tender after being baked. If you want soft onions on the inside, you have to soak them for 30 minutes in ice water, then pat them dry before dipping them. I found them too soft that way.