This soup is my husband's favorite; he always asks for a second helping.

Ladle the soup into oven-safe bowls, place a slice of toasted baguette on top of each, and sprinkle generously with Gruyere cheese.
Place the bowls under the broiler until the cheese is melted and bubbly, about 3-5 minutes. Serve immediately.
Variations & Tips
For a vegetarian version, substitute the beef broth with vegetable broth. You can also experiment with different types of cheese, such as Swiss or Emmental, for a slightly different flavor profile. If you prefer a bit of heat, add a pinch of cayenne pepper to the onions while they caramelize. For a gluten-free option, use gluten-free bread for the topping.