Slice the onions thinly, about 1/8-inch thick; set aside.

Melt butter in a large pot or Dutch oven over medium heat. Add the sliced onions and cook, stirring occasionally, until deeply caramelized and golden brown, about 30-40 minutes.

Pour in the beef broth and sherry, then add bay leaves and thyme sprigs. Bring to a simmer, then reduce heat and gently simmer for 20 minutes.

Remove the bay leaves and thyme sprigs. Taste and season with salt and pepper as needed.

Preheat oven broiler. Ladle soup into oven-safe bowls. Top each with baguette slices and a generous amount of Gruyere cheese.

Place bowls under the broiler and cook until cheese is bubbly and golden brown. Serve immediately.