Start by roasting beef finger ribs under the broiler until browned, about 10–15 minutes. In a large pot, combine ribs with chopped onions, carrots, celery, garlic, bay leaves, and water. Bring to a boil, then simmer uncovered for 4–6 hours; strain the broth.

Using a mandoline, slice onions evenly. In a Dutch oven, melt butter over medium heat. Add onions and cook for 45–60 minutes, stirring occasionally, until caramelized.

Once onions are caramelized, add sliced garlic and thyme, stir in Dijon mustard and Worcestershire sauce, allow to sizzle for 2 minutes. Pour in beef stock and simmer for 15–20 minutes, adjusting seasoning to taste.

Preheat the oven to 400°F (200°C). Slice the baguette, spread with butter, season, and toast for about 10 minutes until golden and crispy. Optionally rub with garlic after baking.

Ladle soup into oven-safe bowls, top with a slice of toasted baguette, sprinkle cheese, and broil for 3–5 minutes until cheese is melted and bubbly.