In a large skillet, heat 1 tablespoon of unsalted butter and a splash of olive oil over medium heat. Add the thinly sliced yellow onions and sauté for 12 to 15 minutes until golden brown and soft.

Clean the skillet to prepare for the au jus. Pour in low-sodium beef broth, Worcestershire sauce, soy sauce, minced garlic, and fresh thyme. Bring to a gentle simmer over medium heat and let it cook for about 5 minutes.

On a clean surface, layer each slice of crusty French bread with Swiss cheese, thinly sliced roast beef, caramelized onions, and another slice of Swiss cheese.

Spread unsalted butter on the outside of each sandwich for a golden crust when grilling.

Heat a non-stick skillet over medium heat. Grill the sandwiches for about 3 to 4 minutes per side until crispy and golden.

Once grilled, remove sandwiches from the skillet and slice in half. Serve with warm au jus on the side for dipping.