Prepare the onion rings

• Peel the onion and slice it into thick rings.
• Separate the rings and set them aside.

Make the batter

• In a bowl, whisk the flour, corn starch, salt, black pepper and garlic powder.
• Pour in the cold sparkling water and mix until just combined and slightly lumpy.

Coat the rings

• Dip each onion ring into the batter, letting the excess drip off.
• Press the rings into the panko breadcrumbs until fully coated.

Fry the onion rings

• Heat the oil to 175 °C (350 °F).
• Fry the rings in batches until golden and crispy, turning once.

Make the dipping sauce

• Mix the Japanese mayo with the ketchup in a small bowl.

Serve

• Drain the onion rings on paper towels and serve hot with the dipping sauce.