In a large pot, combine the beef finger ribs, chopped onion, carrots, celery, smashed garlic, bay leaves, and water. Bring to a boil, then reduce heat to a simmer. Cook for several hours, skimming off the foam. Once done, strain the stock and set it aside.

In a large pot, melt the unsalted butter over medium heat. Add the sliced white and yellow onions along with a pinch of salt. Cook slowly, stirring frequently, until they are golden brown and caramelized—about 45 minutes.

Add the sliced garlic, thyme, Worcestershire sauce, Dijon mustard, and vermouth, scraping up any browned bits from the bottom. Let cook for a few more minutes.

Pour in the beef stock and simmer for 30 minutes. Season with salt and pepper to taste.

Preheat your oven to 350°F (175°C). Spread softened butter on baguette slices and bake until lightly toasted.

Ladle the soup into oven-safe bowls. Top each serving with a slice of toasted baguette and sprinkle generously with Gruyère cheese. Place bowls on a baking sheet and broil in the oven until the cheese is bubbly and golden brown.