01 -
In a large skillet over medium heat, melt 1 tablespoon of unsalted butter with olive oil. Add thinly sliced onions and sauté, stirring regularly, until deeply caramelized and golden brown, about 12 to 15 minutes. Transfer onions to a plate and set aside.
02 -
Wipe out the skillet. In a small saucepan, combine beef broth, Worcestershire sauce, soy sauce, minced garlic, and sprig of thyme. Simmer gently for 5 minutes, then season with salt and black pepper. Strain the mixture through a fine sieve and keep warm.
03 -
Arrange 4 slices of French bread on a work surface. Layer each with 1 slice Swiss cheese, a portion of roast beef, caramelized onions, and a second slice of Swiss cheese. Top each with the remaining bread slices.
04 -
Spread unsalted butter evenly on the outside of each sandwich.
05 -
Heat a clean skillet or griddle over medium heat. Place sandwiches on the surface and grill for 3 to 4 minutes per side, pressing gently, until bread is crisp and golden and cheese is melted.
06 -
Slice sandwiches in half and serve immediately with warm au jus on the side for dipping.