My favorite winter soup is the French classic Soupe à l'Oignon Gratinée. My version has a little vanilla in the recipe. That evolved from a recipe I saw back in the 1970s that used vanilla in a lobster sauce. The chef, Alain Senderen, was a disciple of the Nouvelle Cuisine. The cooking style was a break from the rigid classical French cuisine codified by master such as Escoffier and Careme.
The thought behind the vanilla is that the onions become sweet as they are slowly caramelized and the vanilla lends another flavor level to the soup. Here is my recipe. I served it with a salad for a light dinner.