Peel the large sweet onion and slice it into 1/4-inch thick rings, separating each ring gently. Set aside.
In a deep fryer or large pot, pour in the vegetable oil and heat to 350°F (175°C). Use a deep-fry thermometer for accuracy.
In a mixing bowl, whisk together all-purpose flour, baking powder, and salt.
In another bowl, whisk together buttermilk and egg until well combined. Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Place the panko breadcrumbs in a shallow dish for coating.
Dip each onion ring first into the batter, allowing excess to drip off, then coat with panko breadcrumbs, pressing gently to adhere.
Carefully add onion rings to the hot oil in batches, avoiding overcrowding. Fry for 3-4 minutes or until golden brown and crispy.
Remove onion rings with a slotted spoon and place on a wire rack to drain excess oil. Season immediately with salt and serve warm with your favorite dipping sauce.