Cut off the ends of the onions and peel off the outer layers. Slice the onions into 1/2" rings. Discard the smaller pieces or save for a later use. Separate the rings and add them to a 9x13 baking dish. Pour the buttermilk on top, making sure all of the onion rings are covered with the buttermilk. Cover and place onion rings in the refrigerator to soak for at least 2 hours.
Preheat oven to 400 degrees. Line two large baking sheets with parchment paper and set aside. Remove the onion rings from the fridge and set aside.
Add the flour, smoked paprika, salt, and the pepper to a small bowl and mix well. In a separate bowl, beat the eggs and set aside. In another bowl, add the panko breadcrumbs plus a little extra salt, if desired.
Remove each onion ring from the buttermilk and dip into the flour mixture, then the egg, then the panko, making sure to coat well. Depending on how much you use, you may need more panko bread crumbs. If so, just add a bit more to the bowl.
Place the onion rings in a single layer on the prepared baking sheets and spray the onion rings well with nonstick cooking spray.
Bake at 400 degrees for about 15 minutes. Remove from oven, flip the onion rings, and spray the other side with nonstick cooking spray. Return to the oven and bake for an additional 15 minutes.
Remove from the oven and serve warm.