This is really Swiss in origin, not German (but there was no Swiss cuisine, so I reluctantly selected German because it wouldn't let me leave that space blank). Soups are popular in Switzerland. And the secret to the balance of flavors of this traditional Swiss soup is gently toasting the flour first. It's an extra step that goes a long way in imparting a wonderful depth of flavor to the soup.
In a heavy saucepan over medium heat, heat the flour gently until it begins to turn pale golden brown, stirring constantly. Remove from heat.
In another pan, saute onion slices in butter. When transparent, blend them into the flour. Gradually add beef stock, wine, salt and pepper to taste. Cook gently, stirring frequently, for about 30 minutes.
Put a slice of bread in each of four soup bowls, topping each bread slice with a generous amount of cheese. Using a ladle, pour soup into each bowl, and serve at once.