Cut away any large white ribs from 8 red cabbage leaves. Tear the leaves into 1 1/2-inch pieces. Working in batches, arrange the cabbage pieces in a single layer on a flat microwave-safe plate coated with cooking spray. Mist the cabbage with cooking spray, then sprinkle with salt and mustard powder. Microwave until slightly browned, about 5 minutes. Transfer the chips to a rack to cool. If they're still soft, return to the plate and microwave 30 more seconds.