Peel and remove outermost layer of onion rounds; separate the slices into individual rings. Discard the outermost layers and the innermost smaller rings smaller than 2 inches in diameter, or reserve for another use. Peel off and discard the translucent membrane located inside each ring.

Place ¾ cup flour in a shallow dish; set aside. In a second shallow dish, whisk together buttermilk, eggs, hot sauce, garlic powder, dry mustard, baking powder, 1 teaspoon salt, 2 teaspoons onion powder, 1 teaspoon paprika, ½ teaspoon black pepper, and remaining ½ cup flour until smooth. In a third shallow dish, stir together cereal and 1 ¼ teaspoons salt.

Dredge the onion rings in flour; shake off excess flour, and transfer to a large plate. Working with 1 onion ring at a time, dip in buttermilk mixture. Using a fork, turn onion ring until evenly coated, and lift out of buttermilk mixture, letting excess drip off. Repeat dredging process with flour and buttermilk mixture 1 more time. Transfer dredged onion ring to cereal mixture, turning and pressing gently to adhere; place on a baking sheet. Repeat process with remaining onion rings, flour, buttermilk mixture, and cereal mixture.

Preheat air fryer to 375°F for 5 minutes. Coat all sides of onion rings generously with cooking spray. Working in batches, place onion rings in a single layer in the air fryer basket, and cook until golden and crisp, 11 to 14 minutes, flipping after 8 minutes. Transfer to a plate, and repeat process until all onion rings are cooked.

While onion rings cook, stir together MSG (if using), remaining ¾ teaspoon salt, remaining ¼ teaspoon onion powder, remaining ¼ teaspoon paprika, and remaining ⅛ teaspoon black pepper. Sprinkle hot onion rings with seasoning mixture. Serve hot.