Peel and slice the onions into 1/3- to 1/2-inch rounds. Separate the onions rings and place them in a baking dish (or a large ziplock bag.) Pour the buttermilk over the onion rings, cover, and refrigerate for 4 to 12 hours to marinate.
Once the onion rings have had a long soak in buttermilk, preheat the oven to 450 degrees F and line several baking sheets with parchment paper.
Mix the panko, cornmeal, flour, Old El Paso Taco Seasoning, and salt in a pie pan or shallow dish. In another pie pan, whisk the egg whites until they are very frothy.
Dip the onion rings in the egg whites, followed by the seasoned breading. Make sure to coat them thoroughly. Then lay them on the baking sheets. Whisk the egg whites again if they deflate.
Spray the tops of the onion rings with cooking spray. Bake for 12-15 minutes until very crispy and golden.
NOTE: The longer the onions soak in the buttermilk the more tender they will be after baking. Sometimes I prep them the night before so they have 12-18 hours to soak. Onions that haven't soaked very long will seem rubbery.