Ingredients:

  • 2 large yellow onions, thinly sliced
  • 2 tbsp unsalted butter
  • 1/4 tsp salt
  • 1 tsp granulated sugar
  • 1/2 tsp fresh thyme
  • 2 tbsp dry sherry
  • 1/4 cup beef bone broth
  • 4 slices thick-cut sourdough bread
  • 1 cup grated Gruyère cheese
  • 1/2 cup grated aged Gouda cheese
  • 3 tbsp unsalted butter, softened
  • 1 tbsp Parmesan cheese, finely grated

Instructions:

  1. Melt 2 tablespoons of butter in a heavy-bottomed skillet over medium-low heat. Add sliced onions, salt, and sugar. Cook, stirring occasionally, for 30 minutes until onions are soft and mahogany-colored. Deglaze with water if onions stick.
  2. Toss in 1/2 tsp fresh thyme and pour in 2 tbsp dry sherry. Scrape the bottom of the pan to get all those tasty bits.
  3. Pour in the beef bone broth. Increase heat to medium and stir constantly until the liquid evaporates and forms a thick jammy paste around the onions. Remove from heat.
  4. In a small bowl, combine softened butter and grated Parmesan. Spread this mixture on one side of each bread slice.
  5. Place two slices of bread butter-side down in a clean skillet. Layer with a portion of the cheese blend, a heap of the caramelized onion mixture, and the remaining cheese. Top with the remaining bread slices, butter-side up.
  6. Grill over medium heat for 3-4 minutes per side. Cover the skillet briefly with a lid to ensure the cheese is fully melted and the crust is golden and shattering.