French onion soup is a restaurant classic for a reason; it's an "everyone's favorite," but it can be time-consuming to make at home. That's why making it in a slow cooker will save you the trouble of laboring over the stovetop. All you need to do is set these ingredients in the slow cooker and start the timer.
For the first several hours in the slow cooker, the onions develop a deep caramelized flavor. Then add the broth, sherry and seasonings and let the slow cooker merge the flavors for another hour. If you really want to get that true "pick the broiled cheese off the side of the mug" feeling that you get with restaurant-served French onion soup, just ladle the soup into ovenproof mugs, add the cheese and croutons, and pop it all under the broiler!