This fabulous soup can be served at the beginning of a meal, or it can be an entire meal all by itself (with some good bread and a salad on the side). Don't hurry the onions - let them brown slowly in order to get a rich broth.
3 medium onions, thinly sliced
2 tbsp butter
6 cups beef broth, (reconstituted canned is fine)
4 thick slices French bread, or Italian bread, toasted
1 cup shredded Swiss, or mozzarella
In a medium saucepan, s-l-o-w-l-y sauté the sliced onions in the butter until golden brown. This will take longer than you'd expect - maybe 30 minutes or so. Don't try to rush it. Add the beef broth and simmer, covered, for about 20 minutes. Add salt and pepper - but go easy because canned broth tends to be pretty salty already.
Ladle the soup into 4 ovenproof bowls. Float a slice of toasted bread in each one, and sprinkle evenly with the grated cheese. Put the bowls on a baking sheet, turn the oven to broil and cook for a couple of minutes, until the cheese melts and gets bubbly.