taken from The Low-Carb Comfort Cookbook by Eades
(with a minor alteration)

1 1/2 C thinly sliced onion (sweet)
4 TBLS butter
1 TBLS Wondra flour
3 1/2 C low-carb beef or mushroom broth
salt and pepper to taste

Low carb bread or croutons or thinly sliced low carb rolls
1/2 C swiss cheese

After this soup has finished simmering, preheat oven to 375 F. Have ready lightly buttered ovenproof earthenware soup bowls or a deep casserole dish.

Heat butter in a heavy, medium saucepan. Add onions and cook until translucent. Add the flour and stir. Add the broth and stir. Simmer for about 30 min. Season with salt and pepper. Put the soup in a the prepared bowls.

Toast bread, or thinly slice rolls, discarding ends and toast slices in toaster oven. Place on top of soup. Sprinkle cheese on top. Do not cover. Put the bowls in the oven for about 15 min until cheese and bread look crusted and slightly browned. Serve at once.

Makes 5 or 6 servings.
Approximately 32g carb and 4g fiber for a net total carb of 28g. (Before bread counts as that may vary but it does include cheese)
So 5.6g (at 5 servings) or 4.67g (at 6 servings). (Again before bread)

Thanks for posting this...I've been missing French Onion Soup for weeks now, and I knew it was just going to get worse when we got to the cool nights of Fall. My standard recipe calls for port wine, and was resisting my best efforts to LC modify it...this sounds like it may do the trick!

Not as good as port wine, but I like to make it with a combo of cognac and red wine. It gives it a deep rich flavor. I also cook mine for about 5 hours. I havent made a LC version of it, but most alcohols arent high carb (port is :-(... and I love it too) but eventually when I am in the mood I will experiment with it. I have yet to find low carb broth though. Everything I find is 3g per cup, so I might need to make my own from scratch.