In a skillet, melt 1 tablespoon of butter with olive oil over medium heat. Add the sliced onions and cook, stirring frequently for 12-15 minutes, until they turn a deep golden brown and become sweetly fragrant.

In a small saucepan, combine beef broth, Worcestershire sauce, soy sauce, minced garlic, and thyme. Simmer for about 5 minutes to blend the flavors, then season with salt and pepper and strain if desired.

Layer each slice of crusty French bread with a generous amount of Swiss cheese, followed by tender roast beef and the caramelized onions. Top off with another slice of Swiss cheese and close the sandwich with remaining bread.

Spread unsalted butter on the outsides of each sandwich. Grill them on medium heat for 3-4 minutes per side until they’re golden brown and the cheese has melted, creating that inviting crispiness.

Cut the sandwiches in half and serve them immediately with the warm au jus for an irresistible dipping experience.