Preheat oven to 375 degrees.

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 crumbs with the spices and transfer to a plate or bowl. Set aside.

Fill a small bowl with egg substitute. Prepare a baking sheet (or two) by spraying with nonstick spray. Set aside.

Pop-Up Tip! For the following step, use tongs or a fork to dip the rings into the egg substitute and cereal crumbs -- it'll keep your fingers from getting eggy & crumb-covered. (Don't pierce the rings with the fork; just balance them on it.)

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).

Bake in the oven until outsides are crispy and insides are soft, 20 - 25 minutes, carefully flipping rings over about halfway through. Serve with mustard, if using. Enjoy!