Preheat the oven to 375 degrees F.
Slice the ends off the onion and remove its outer layer. Cut onion into 1/2-inch-wide slices, and separate into rings. Set aside.
Using a blender or food processor, grind cereal to a breadcrumb-like consistency. Season the crumbs with spices and transfer to a plate or bowl.
Fill a small bowl with egg substitute. Prepare a baking sheet (or 2) by spraying with nonstick spray. Set aside.
One at a time, dunk each ring in egg substitute, give it a shake to remove any excess, and then coat it in the seasoned crumbs. Evenly place rings on the baking sheet(s).
Cook's Note: Use tongs or a fork to dip the rings into the egg substitute and cereal crumbs. It'll keep your fingers from getting eggy and crumb-covered. Don't pierce the rings with the fork; just balance them on it.
Bake in the oven until the outsides are crispy and insides are soft, 20 to 25 minutes, carefully flipping rings over about halfway through. Serve with mustard, if using. Enjoy!
PER SERVING (entire recipe): 155 calories, 1g fat, 515mg sodium, 41g carbs, 16g fiber, 7g sugars, 9g protein