French onion soup is a classic comfort dish built on deeply caramelized onions and a rich beef broth. The secret to its signature dark color and sweet, savory flavor lies in cooking the onions low and slow until they turn a deep golden brown. A touch of dried thyme and a bay leaf add an earthy background note to the broth. It is finished with thick slices of toasted French bread and a generous layer of melted Gruyere and Parmesan cheese. Serve it on a chilly evening when you need a warming, satisfying meal.