Ingredients:

  • 3 lbs yellow onions, thinly sliced
  • 4 tbsp unsalted butter, cubed
  • 1 tsp kosher salt
  • 1/2 tsp granulated sugar
  • 6 cups high-quality beef stock
  • 1/2 cup dry white wine
  • 2 tbsp balsamic vinegar
  • 1 tbsp Worcestershire sauce
  • 3 sprigs fresh thyme
  • 2 bay leaves
  • Black pepper to taste
  • 1 French baguette, sliced into 1-inch rounds
  • 2 cups Gruyère cheese, freshly shredded
  • 1/4 cup Parmesan cheese, finely grated

Instructions:

  1. Slice the onions. Cut 3 lbs of yellow onions into thin, uniform rounds or half moons.
  2. Layer the slow cooker. Place the onions in the basin and toss with 1 tsp kosher salt, 1/2 tsp granulated sugar, and 4 tbsp cubed unsalted butter.
  3. Caramelize the base. Cover and cook on LOW for 8 hours until the onions are deep brown and jammy.
  4. Deglaze the pot. Pour in 1/2 cup dry white wine and use a wooden spoon to scrape any browned bits (fond) from the sides.
  5. Build the broth. Add 6 cups beef stock, 2 tbsp balsamic vinegar, 1 tbsp Worcestershire sauce, 3 sprigs fresh thyme, and 2 bay leaves.
  6. Simmer for depth. Cover and cook on LOW for an additional 2 hours until the flavors are fully married.
  7. Season the soup. Remove the thyme sprigs and bay leaves, then add black pepper to taste and adjust salt if necessary.
  8. Prepare the crostini. Slice the baguette into 1 inch rounds and toast on a baking sheet at 400°F for 5 minutes until golden and crisp.
  9. Assemble the bowls. Ladle the soup into oven safe crocks, top with a toasted baguette slice, and pile on the shredded Gruyère and Parmesan.
  10. Broil the topping. Place the crocks on a baking sheet and broil for 2-3 minutes until the cheese is bubbly and browned.