For the Soup:

1. 1. Caramelize the Onions: In a large pot over medium heat, melt the butter. Add thinly sliced yellow onions and cook slowly for about 30 minutes until golden brown and fragrant, stirring occasionally.
2. 2. Add Broth: Pour in the beef broth and season with salt and pepper to taste. Bring to a gentle boil, then reduce heat to low and simmer for another 20 minutes.

For the Baguette Topping:

1. 3. Prepare the Baguette: Preheat your oven to broil. Slice the baguette and lay on a baking sheet. Toast under the broiler until golden brown, keeping an eye on them to prevent burning.
2. 4. Assemble Your Soups: Ladle hot soup into oven-safe bowls or crocks. Place toasted baguette slices on top, and generously sprinkle with grated Gruyère cheese.
3. 5. Broil Until Golden: Place bowls under the broiler until cheese bubbles and turns golden brown—about 3 to 5 minutes.