Halve, peel and thinly slice the onions. Melt the butter in a saucepan, add the onions and sauté until onions turn translucent.
Quarter, core, peel and thinly slice the apple. Combine it with the onions and sauté for a few minutes.
Gradually add the flour, then the beef stock and beer. Season to taste with marjoram, salt and pepper. Simmer over medium-low heat for 15-20 minutes.
Before serving sprinkle the soup with chives, grated Emmenthal and/or croutons.