Heat the oven to 425°F and line a baking sheet with parchment. Separate the onion into matching pairs of rings.
Stir the mashed avocado, tomato, red onion, lime juice, cilantro, and 1/4 teaspoon salt into a chunky guacamole.
Spoon guacamole between two matching onion rings, leaving a narrow border so the filling stays enclosed. Repeat with the remaining rings and chill for 10 minutes.
Set out the flour, beaten eggs, and panko mixed with garlic powder, pepper, and the remaining salt.
Coat each stuffed ring in flour, dip it in egg, then press it into panko until covered. Place the rings on the prepared sheet and spray lightly with oil.
Bake for 12 minutes, turn carefully, and bake 10 to 13 minutes more until the coating is crisp and the onion is tender.
Cool for 5 minutes and serve with lime wedges or a simple chipotle dip.