If salt is not a concern, I really like the "diet cabbage soup" variations even when not on a diet.
1 packet french onion soup mix (lipton is vegan btw)
1 can crushed tomatoes
1 large bag shredded cabbage (you don't want the tiny chop)
Celery (I used about half a bunch)
Onion (one large yellow or white)
other vegies you might have around
enough water to cover, plus maybe a little more (probably 6-8 cups)
Just boil it all up and cook for maybe 15 minutes or until the veggies are tender