Ingredients:

  • 6 large yellow onions, thinly sliced (approx. 3 lbs / 1.36 kg)
  • 2 tbsp unsalted butter
  • 1 tbsp extra virgin olive oil
  • 1/2 tsp sea salt
  • 4 cloves garlic, minced
  • 8 cups low-sodium beef bone broth
  • 1/2 cup dry white wine
  • 2 sprigs fresh thyme
  • 1 bay leaf
  • 1 French baguette, sliced into 1-inch rounds
  • 6 oz Gruyère cheese, shredded
  • 2 oz Parmesan cheese, finely grated

Instructions:

  1. Melt the butter and olive oil in a 6-quart cast iron Dutch oven over medium-low heat. Add the sliced onions and salt. Stir occasionally for the first 10 minutes to soften.
  2. Increase heat to medium and cook for 45–60 minutes. Stir every 5–8 minutes, scraping the bottom of the pot whenever a brown film (fond) forms, until onions are a deep mahogany color.
  3. Stir in the minced garlic and cook for 60 seconds until fragrant.
  4. Pour in the white wine or sherry and deglaze the pot, scraping up the fond. Let the liquid simmer until it has reduced by half.
  5. Pour in the beef bone broth. Add the thyme sprigs and the bay leaf. Bring to a boil, then reduce heat to low, cover partially and simmer for 20 minutes.
  6. Remove the bay leaf and thyme stems. While the soup simmers, toast baguette slices in the oven until crisp.
  7. Ladle the hot soup into broiler-safe crocks. Place toasted bread slices on top and cover generously with shredded Gruyère and Parmesan.
  8. Place the crocks under the broiler for 2–4 minutes until the cheese is bubbling and charred in spots.