01 - Preheat oven to 425°F. Line two baking sheets with parchment paper and lightly spray with cooking spray.
02 - Peel onions and cut into 1/2-inch thick rings. Separate into individual rings, discarding very small inner ones.
03 - Arrange three shallow bowls: whisk flour, salt, pepper, garlic powder, onion powder, and smoked paprika in the first; beat eggs and buttermilk in the second; combine panko breadcrumbs with grated Parmesan in the third.
04 - Dip each onion ring into the flour mixture, shaking off excess; then submerge into the egg-buttermilk mixture, letting excess drip off; finally, dredge in the panko-Parmesan mixture, pressing gently to adhere.
05 - Place coated onion rings in a single layer on prepared baking sheets. Lightly spray tops with cooking spray.
06 - Bake for 10 minutes, flip each ring, then bake an additional 8–10 minutes until golden and crispy.
07 - Remove from oven and serve hot.