Les Halles may be a bit of a walk from metro center, but they have an excellent onion soup (the recipe is on the site here and I just can't make it taste as good, but I am trying) and it isn't very pricey at all!
How about Bistro D'Oc. It's near the Metro Center metro stop. I'm almost positive they are open for lunch and their french onion soup is delicious. Try it with the goat cheese salad and you will be more then happy.