In a large dutch oven over medium heat, melt the butter.
4 tablespoons unsalted butter
Add in the onions, salt, and black pepper. Cook, stirring often until the onions are caramelized, about 30 - 40 minutes.
5 large yellow onions, 1 teaspoon kosher salt, 1/4 teaspoon black pepper
Add the sherry, deglazing the pan. Then stir in the beef broth. Add the thyme and the bay leaf and bring the mixture to a boil. Reduce to a simmer and cook, uncovered, for 20 minutes.
1/2 cup sherry, 6 cups beef broth, 1 teaspoon dried thyme, 1 bay leaf
Taste the soup and add more salt or pepper as needed.