A more traditional way is to serve with croutons that are browned basted with Olive oil and rubbed with garlic and perhaps sprinkled with cheese. It can also be served without the cheese on the croutons and the cheese served on the side.
I prefer to serve it this way. the cheese does not congeal to a large hard to separate mass on top of the bowl, making it easier to eat each spoonful of soup without having to cut and untangle the cheese. Plus you can add more cheese as you please.
Traditional recipes call for the addition of vermouth or sherry, and a finishing touch of cognac. While they certainly do enhance the flavor of the soup they aren't an absolute necessary item. Well cooked onions and a good beef stock cooked properly will suffice. The recipe to follow is for 4 servings.
Also the cheese can certainly be varied. Tradition is for Swiss or Parmesan. You can use Gruyère, and in this recipe I used a combination of grated Asiago and Parmesan.