Spray a baking sheet slightly with oil and set aside. Place bread crumbs in bowl add cayenne, mix, and then seperate into 2 bowls set aside. In another bowl, combine buttermilk, 2 tablespoons of flour, salt and pepper and set aside. Slice onions into 1/2-inch circles and separate into rings, keeping only large, whole rings (reserve rest of onions for other uses). You should come out with about 12 to 14 rings.
Place the remaining flour in a sealable plastic bag, then add onions, and shake to coat. Dip onions 1 at a time into the buttermilk mixture, then dip into bread crumbs and place on baking sheet. Spray canola oil evenly over rings and bake for 20 minutes, or until coating is crisp. Season with salt, to taste, and serve immediately.