Now for *red* onion substitutes, all kinds of things would be fine - celery, celeriac, fennel, radishes... Red onions have a lot of final flavor, so something with a lot of flavor makes a good substitute. It's white onions that are so mild that it's hard to find a similar mild-texture-no-flavor equivalent.
Though I should probably be trying jicama for that. It's nearly flavorless. Anybody know if it stays crunchy after cooking?