Preheat the oven to 400°F (200°C), or turn on the broiler.
Peel the onion and slice into rings, about ⅕ inch thick.
Mix the dry ingredients in a bowl. Whisk the egg in another bowl.
Dip the onion rings in the egg batter and then in the flour mix, one at a time.
Place the rings on a baking sheet covered with parchment paper.
Drizzle or spray oil on the rings and bake in the oven for 15–20 minutes. If you are using the broiler, keep a close eye on them; they're done when golden brown and crisp.
To get the smell of onion off of your hands, rub your fingers with lemon juice or vinegar.