Peel and slice the onions into ½-inch thick rings. Separate the rings and set aside.
In a shallow dish, combine the flour, baking powder, garlic powder, paprika, salt, and black pepper.
In another shallow dish, whisk together the milk and beaten egg making sure it's well-blended.
Spread the panko bread crumbs on a large plate or flat baking dish.
Dip each onion ring into the flour mixture, making sure it's fully coated. Shake off any excess flour. Next, dip the onion ring into the milk and egg mixture, allowing any excess to drip off. Finally, coat the onion ring in breadcrumbs, pressing gently to ensure they adhere. Place the breaded onion rings on a plate or wire rack.
In a deep fryer or heavy-bottomed pot, heat the oil to 365°F (185°C). If you don't have a thermometer, you can test the oil by dropping a small pinch of flour into it. If it sizzles and bubbles immediately, the oil is ready.
Carefully lower the breaded onion rings into the hot oil in batches, being careful not to overcrowd the fryer or pot. Fry for 2-3 minutes, or until the onion rings are golden brown and crispy. Use a slotted spoon or tongs to remove the onion rings from the oil and transfer them to a paper towel-lined plate to drain any excess oil.
Allow the onion rings to cool for a minute or two before serving. Enjoy them with your favorite dipping sauce, such as ketchup, ranch, or barbecue sauce.
To maintain their crispy texture, consume the onion rings soon after frying.