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 waiting for things to cook. All you need to do is set these ingredients in the slow cooker and start the timer.
The onions will develop even more flavor while adding to the broth, as the entire thing cooks away for several hours. And if you really want to get that true "pick the broiled cheese off the side of the mug" feeling that you get with true restaurant-served French onion soup, just ladle the soup into ovenproof mugs before adding the cheese and croutons and popping it all under the broiler!