American French Onion Soup

Preheat oven to 425 degrees F (220 degrees C).

Melt 1/4 cup butter in a large, oven-safe skillet on medium heat. Stir in onions until they are all coated in butter. Transfer skillet to the preheated oven and cook onions, stirring occasionally, until they are tender and well browned, about 1 hour.

Transfer skillet to the stovetop; cook and stir onions over medium heat until they start to brown and stick to the bottom of the pan, about 5 minutes. Pour sherry and vinegar into the pan, and bring to a boil while scraping browned bits of onion off the bottom of the pan with a wooden spoon.