The secret to French onion soup is lots of love and attention, hours and hours of it. (Scroll down below for the full recipe.) I once spoke to a chef about how a restaurant's French onion soup was made. I learned that it took all day, from morning to night, just to caramelize the onions. They would slice 44 quarts of onions that then went into a giant army pan on two burners. That entire 44 quarts reduced into 8 quarts once caramelized. Throughout the day, everyone in the kitchen would "take care" of the caramelizing onions. This recipe made in the slow cooker gives the onions the time they need to caramelize without demanding too much attention.
Traditionally, sugar is not something used in caramelizing onions. True caramelized onions don't need sugar because you draw out the natural sugars from the onions through the slow-cooking process — allowing them to build flavor on the pan and then deglazing with just a touch of liquid, adding in a bit of butter and then deglazing again. This is repeated all day long.
Most of us don't have time to tend to caramelizing onions all day long. In this recipe, we use brown sugar and butter to help the caramelizing process. The slow cooker holds a lot of steam, so it helps deglaze the onions all on its own. In the end, you have a delicious French onion soup that rivals any other. Use a good-quality no-sodium beef broth or stock, with no added spices or flavors. The broth you use in this recipe will have a great effect on the finished product.