I made the following french onion soup - minus the bread - this past weekend. Even DH, a high carber, loved it.

1 thinly sliced onion (~ 1 cup)
1/2 cup mushrooms
2 tablespoons butter
1 tablespoon worcestershire sauce
2 cans beef stock (0 carbs)
1-2 Beef Bouillon Cubes
1 teaspoon black pepper
cheese of your choice for topping - mozz, swiss, parm.

Sautee onions and mushrooms in butter until tender. Add beef stock and wochestershire sauce. Once hot, add beef bouillon cubes until dissolved. Season to taste with black pepper. Pour into bowls and add cheese to top, place in oven at 350 until cheese is melted and bubbly.