Peel and cut the onions into rings, carefully separate them, set aside.
In a medium bowl, stir together the flour, baking powder, onion powder, garlic powder, paprika, and salt.
Dip the onion slices into the flour mixture until they are all coated and set aside on a cutting board.
Next, using a fork, whisk the egg and milk into the flour mixture.
Add the Panko breadcrumbs to a shallow dish, set aside.
Dip the floured rings into the batter to fully coat, place on a wire rack to drain until the batter stops dripping.
Heat the oil in a deep-fryer or large pan to 365 degrees F.
Dip the onion rings one at a time into the Panko crumbs, getting the crumbs all over the ring to coat.
Deep fry the rings a few at a time for 2 to 3 minutes, or until golden brown. Drain excess oil by placing them on the wire rack after frying.
Season with salt if needed, and serve.