Almost impossible to answer this question!
It depends how much cheese is on top, and how much bread is soaking in the broth. Unless you make it yourself, or consume it at a chain-restaurant where the meal portions are very standardized and the nutrition information available, you will have to make your best guess. Sorry, this is not a very helpful answer, but sometimes "guess" is the best we can do!
Ok for comparison, french onion soup "with cheese and croutons" comes in at around 550 calories at Ruby Tuesdays, 230 calories at Panera, and 388 at Applebees. The kind you can take home and heat from Costco or Trader Joes is more like 210. If you are eating it in a restaurant, I would err on the side of caution and assume it'll be in the 350-500 range.
Edited by: BUNNYKICKS at: 3/18/2013 (17:53)