Peel onions and cut into rings. Heat half the butter and sauté onions until soft and translucent. Add broth and season with salt and pepper. Simmer for 10 minutes.
Soak the Zwieback briefly in the dark beer. Heat the remaining butter in a frying pan and fry the Zwieback on both sides till golden brown.
Serve the soup in bowls, topped with the browned Zwieback toasts and chopped chives.
(As an alternative to beer, sprinkle the Zwieback with celery and onion salt, top with cheese and grill in the oven before adding to soup.)
Prep Time under 25 minutes