Prepare the onions: Separate the sliced onion rings. If any are too small, you can discard them or use them as smaller pieces.

Set up the dredging stations: In a shallow bowl, whisk together the flour, cornstarch, salt, pepper, paprika, and garlic powder. In a second shallow bowl, whisk together the milk and egg until well combined.

Coat the onion rings: Dip each onion ring into the flour mixture, ensuring it's fully coated. Shake off any excess. Then, dip the coated ring into the milk-egg mixture, allowing any excess to drip off. Finally, coat the ring generously with panko breadcrumbs, pressing gently to adhere.

Heat the oil: Pour about 2-3 inches of vegetable oil into a deep fryer or heavy-bottomed pot. Heat the oil to 375°F (190°C). Use a thermometer to ensure the correct temperature.

Fry the onion rings: Carefully place a few coated onion rings into the hot oil, being careful not to overcrowd the pot. Fry for 2-4 minutes, or until golden brown and crispy, turning occasionally.

Drain and cool: Using a slotted spoon or spider strainer, remove the fried onion rings from the oil and place them on a wire rack set over a baking sheet to drain any excess oil and maintain crispiness.

Repeat and serve: Continue frying the remaining onion rings in batches. Serve immediately while hot and crispy.