Preheat oven to 450 degrees. Line a large baking sheet with foil and lightly spray the foil with cooking spray. Set aside. In a wide bowl stir use a fork and together the egg and mayonnaise until fully combined. Pour one third of the dry mixture into a wide bowl. Set aside.
In a medium bowl stir together the dry ingredients until fully combined. Set aside. cut the onion into desired size slices then separate each slice into rings. Place the rings one at a time into the egg mixture and turn to fully coat.
Place the coated rings into the dry mixture and turn to fully coat. Add more dry mixture as needed. Place the coated rings onto the prepared baking sheet. Bake at 450 degrees for 12 to 15 minutes or until golden.
Remove from the oven and cool on the pan a couple of minutes. Serve warm as a side dish or an appetizer.