• Line a large baking sheet with parchment paper and set aside.
• Add the buttermilk to a medium bowl.
• In a separate medium bowl, whisk together the flour, garlic powder, kosher salt, and black pepper.
• Slice the onion into ½-inch rings, separating each ring.
• Dip each onion ring into the buttermilk, then coat it in the flour mixture. Place the coated rings onto the prepared baking sheet. Repeat with the remaining onion rings.

Frying

• Fill a large pot or Dutch oven with 1-2 inches of vegetable oil, or enough to fully submerge the onion rings. Heat the oil over medium heat until it reaches 375°F.
• While the oil heats, line a baking sheet with paper towels.
• Fry the onion rings in small batches, about 2-3 at a time, for about 3 minutes, flipping halfway through, until golden brown.
• Using a slotted spoon, remove the onion rings and place them on the paper towel-lined baking sheet to drain.
• Repeat with the remaining onion rings.
• Serve hot with fry sauce, ketchup, or ranch dressing.