Cut the onions in half at the stem. Lay the halves down flat and cut thin slices.
In a large soup pot, melt the butter over low heat, then add the onions, turn
up the heat to medium and sauté. After a minute or so add the flour and
continue to sauté until the onions have a nice golden color.
Add the beef broth and season with salt and pepper. (Unless you bought low-sodium broth, you won't need much salt.) Bring to a boil.
Once the soup has boiled, turn the heat down and let the onions cook for about 15 minutes. Stir occasionally. Preheat the broiler now.
Once the soup has cooked, taste and adjust for seasonings with salt and pepper. The soup is now ready for the broiler.
Next steps:
Grate the Gruyere cheese and set aside.
Once the broiler is heated and the soup is done cooking, ladle the onion soup into the soup crocks. Top with grated cheese. Broil for 2 minutes or until the cheese bubbles and browns slightly.
Serve immediately. Watch out for the crocks; they will be hot!