Peel the onions and slice them into 1/4-inch thick rings. Separate the rings and set aside.
In a shallow dish, mix flour, garlic powder, smoked paprika, salt, and pepper. In another dish, pour in the buttermilk. In a third dish, combine panko breadcrumbs and grated Parmesan cheese.
Dip each onion ring into the flour mixture, then into the buttermilk, ensuring it's well-coated. Finally, dip it into the panko-Parmesan mixture, pressing lightly to ensure it adheres well.
In a large, deep skillet or pot, heat about 2 inches of oil over medium-high heat until it reaches 350°F (175°C). You can test if the oil is ready by dropping in a small piece of bread; if it sizzles and browns quickly, it’s ready.
Carefully add a few onion rings to the hot oil, being sure not to overcrowd the pan. Fry for 2-3 minutes or until they turn golden brown, flipping halfway through for even cooking.
Once cooked, remove the onion rings with a slotted spoon and drain on a paper towel-lined plate. Sprinkle with additional salt if desired and repeat with the remaining onion rings.
Serve them warm with your favorite dipping sauce such as ranch, garlic aioli, or ketchup.