Heat 1 tablespoon of unsalted butter with a splash of olive oil in a skillet over medium heat. Add yellow onions and stir frequently for 12-15 minutes until caramelized.

In the same skillet, add beef broth, Worcestershire sauce, soy sauce, minced garlic, and thyme. Simmer for 5 minutes then strain the mixture to create au jus.

Layer Swiss cheese on the bottom slice of French bread, followed by roast beef and caramelized onions. Add more Swiss cheese and top with another slice of bread.

Spread unsalted butter on the outer sides of each sandwich. Optionally, sprinkle with salt.

Preheat a skillet and grill the sandwiches for about 3-4 minutes per side until golden brown and cheese melts.

Slice the sandwiches in half and serve warm with the prepared au jus on the side.