Slice the onion into ¼ -inch slices and separate the rings.
Set up a breading station. Place the flour in a dish and mix in ¼ teaspoon salt and 1/8 teaspoon pepper. Pour the buttermilk into a bowl. Place the breadcrumbs in a mini food processor to grind them up finely. Alternatively, you can place them in a plastic bag and then crush them with a rolling pin. Grinding them up finely will help them adhere to the onions. Mix the breadcrumbs in another dish with the chili powder, garlic powder, cumin, cayenne, ½ teaspoon salt and ¼ teaspoon pepper.
Working with one or two onion rings at a time, place the onion rings in the flour and toss them around to coat them well. Then dip the rings into the buttermilk. Let the excess buttermilk drip off and then place the rings into the breadcrumbs. Turn the rings around in the breadcrumbs until they are fully coated. This will work best if you do a couple of rings at a time otherwise the breadcrumbs will get clumpy. Carefully lift the rings from the breadcrumb mixture and place them on two baking trays lined with parchment paper or aluminum foil.
Spray the onion rings lightly with olive oil spray and place the trays in the oven. Bake about 15 minutes until the onion rings are golden brown. Remove from oven and serve immediately.
For nutritional calculations, I only used the amount of flour, buttermilk and breadcrumbs that adhered to the onion rings. The rest was discarded.