Ingredients:

  • 2 Large yellow onions, thinly sliced
  • 2 tbsp Unsalted butter
  • 1 tsp Balsamic vinegar
  • 0.5 tsp Kosher salt
  • 1 lb high-quality roast beef, thinly shaved
  • 8 slices sourdough bread
  • 8 slices sharp provolone cheese
  • 2 tbsp horseradish aioli
  • 3 tbsp salted butter, softened
  • 2 cups beef bone broth
  • 0.25 cup dry red wine
  • 1 clove garlic, smashed
  • 1 sprig fresh rosemary
  • 1 tbsp Worcestershire sauce
  • 1 tbsp cold unsalted butter

Instructions:

  1. In a heavy skillet over medium-low heat, melt 2 tablespoons of butter. Add sliced onions and salt. Cook for 20-25 minutes until deep mahogany. Deglaze with balsamic vinegar and set aside.
  2. In a small saucepan, combine beef broth, red wine, smashed garlic, rosemary, and Worcestershire sauce. Bring to a simmer and reduce for 15-20 minutes. Strain solids and whisk in 1 tablespoon of cold butter.
  3. Lay out bread slices. Spread horseradish aioli on the interior. Place one slice of cheese on each piece of bread to create a moisture barrier.
  4. On four of the slices, layer the shaved roast beef and caramelized onions. Close the sandwiches and brush the exteriors with softened salted butter.
  5. Preheat panini press to medium-high. Grill sandwiches for 3-5 minutes until the bread is golden-brown and cheese is fully melted. Serve immediately with the warm au jus.