You may be skeptical of using the oven to caramelize the onions, and I get it. It's a brilliant method, though. Instead of standing next to the stove for a long, long time, waiting for the moment of transformation into delicious caramelization, it's so much easier to allow the oven to do the work for you, while you relax or busy yourself with something else.
If you're using store bought Beef Broth, use only 2 cups of beef broth and 4 cups of chicken broth. Store bought beef broth is just seriously lacking in flavor, so it's best to use mostly chicken broth with just a bit of beef broth.
You can prepare the soup ahead of time and store it in the refrigerator until you are ready to serve it. Then simply reheat the soup, top it with the toasted bread and cheese and place it under the broiler just long enough to melt the cheese and serve.