01 - Peel and slice onions into 1/2-inch rings, then separate individual rings and set aside.
02 - In a medium bowl, whisk together flour, cornstarch, baking powder, salt, black pepper, and paprika until evenly combined.
03 - Gradually whisk cold sparkling water into the dry mixture until a smooth and lump-free batter forms.
04 - Pour panko breadcrumbs into a shallow dish for coating the onion rings.
05 - Heat vegetable oil in a deep pot or fryer to 350°F (175°C) for frying.
06 - Dip each onion ring into the batter, allowing excess to drip off, then thoroughly coat with panko breadcrumbs.
07 - Fry the coated onion rings in batches for 2 to 3 minutes, turning occasionally until they are golden and crisp.
08 - Remove fried rings using a slotted spoon and place them on paper towels to drain excess oil.
09 - In a small bowl, combine mayonnaise, ketchup, Dijon mustard, lemon juice, and smoked paprika. Season with salt and pepper to taste.
10 - Serve the hot, crispy onion rings accompanied by the prepared dipping sauce.