Prepare the ingredients: Peel 2-3 medium onions and slice into 1cm thick rings. Separate the rings and set aside. Finely chop 2 tbsp fresh parsley.
Make the wet batter: In a large bowl, mix 1 cup all purpose flour, 2 tbsp corn starch, 1 tsp baking powder, 1 tbsp garlic powder, 1 tbsp paprika powder, 1 tbsp black pepper powder and salt to taste. Slowly whisk in 1 cup cold water until smooth and lump-free.
Prepare the dry coating: In a separate bowl, combine 1 cup all purpose flour, 2 tbsp corn starch, 2 tbsp fresh parsley, 1 tbsp garlic powder, 1 tbsp black pepper powder, 1½ cup panko bread crumbs and salt to taste. Mix well.
Coat the onion rings: Dip each onion ring into the wet batter, letting excess drip off, then coat evenly in the dry mixture. Press gently so the crumbs stick well.
Heat the oil: Pour neutral oil into a deep pot and heat to 170°C-180°C.
Fry until golden: Carefully place coated onion rings into the oil. Fry in batches for 2–3 minutes until golden brown and crispy. Transfer to a rack or paper towel to drain.
Make the spicy mayo: In a small bowl, mix ½ cup Japanese Kewpie mayonnaise, 2 tbsp sriracha sauce and 1 tsp lemon juice until smooth.
Serve: Serve the onion rings hot with the spicy mayo dipping sauce on the side.