Peel the onion and slice it into thick rings.
Carefully separate the rings and set aside.
In a bowl, whisk together flour, paprika, salt, black pepper, and garlic powder.
Gradually add cold water and whisk until you have a smooth, thick batter that can coat the onion rings well.
In a separate bowl, mix breadcrumbs, panko, paprika, salt, black pepper, and garlic powder until evenly combined.
Dip each onion ring into the batter, letting any excess drip off.
Transfer immediately to the breadcrumb mixture and press gently so the coating sticks well.
Repeat with all onion rings and place them on a tray.
Heat oil in a deep pan over medium heat.
Fry the onion rings in batches for 2–3 minutes per side, or until golden brown and crispy.
Remove and drain on a wire rack or paper towel.
Mix mayonnaise and sweet chilli sauce in a small bowl until smooth.
Serve the onion rings hot with the sweet chilli mayo on the side.