Preheat oven to 400ºF. Coat a large baking sheet with cooking spray.
In a large bowl, combine flour, garlic powder, dry mustard and salt. Add onions and toss to coat.
Pour buttermilk into another large bowl, add onions and toss to coat (use your hands for the best result).
Place bread crumbs in another large bowl, add onions and toss to coat. Transfer onions to prepared baking sheet and spray with cooking spray.
Bake until golden brown, about 35 minutes. Serve hot. Yields about 1/2 an onion per serving.
Flavor Booster: Give onion rings more crunch and fiber by using homemade multi-grain bread crumbs. Place 2 slices of toasted multi-grain bread in a food processor or blender and mince using on/off pulses.