Ingredients:

  • 5 lbs yellow onions, thinly sliced
  • 3 tbsp unsalted butter, divided
  • 1 tbsp olive oil
  • 1 tsp kosher salt
  • 4 cups low-sodium beef broth
  • 1 tbsp balsamic vinegar
  • 2 cloves garlic, minced
  • 1 sprig fresh thyme
  • 1/2 tsp black pepper
  • 1 baguette, sliced into 1-inch rounds
  • 6 oz Gruyère cheese, grated
  • 1 tbsp melted butter

Instructions:

  1. Place the sliced onions, salt, and half of the butter into the crockpot.
  2. Set the slow cooker to LOW, cover, and cook for 8 to 10 hours until the onions are deep mahogany-brown.
  3. Stir in the remaining butter, olive oil, minced garlic, and fresh thyme.
  4. Pour in the beef broth and balsamic vinegar, stirring well to incorporate the caramelized bits.
  5. Cover and cook on LOW for another 1 hour to meld flavors.
  6. Preheat the oven broiler and toast baguette slices on a baking sheet until lightly golden.
  7. Ladle the soup into oven-safe ramekins, top with 2 baguette slices and a generous handful of grated Gruyère.
  8. Broil for 2–3 minutes until the cheese is bubbling and charred.