01 -
In a large skillet over medium heat, melt 1 tablespoon unsalted butter with olive oil. Add thinly sliced onions and cook, stirring frequently, until deeply golden and fully caramelized, about 12 to 15 minutes. Transfer onions to a plate and set aside.
02 -
Wipe the skillet clean. In a small saucepan, combine beef broth, Worcestershire sauce, soy sauce, minced garlic, and thyme. Bring to a simmer over medium heat for 5 minutes. Season with salt and black pepper to taste. Strain the liquid through a fine sieve and keep the au jus warm.
03 -
Arrange 4 slices of French bread on a clean work surface. Layer with 1 slice of Swiss cheese, generous portions of roast beef, caramelized onions, and another slice of Swiss cheese. Top with the remaining bread slices.
04 -
Spread unsalted butter evenly over the exterior sides of each assembled sandwich. Heat a clean skillet over medium-low. Grill sandwiches in batches until bread is golden and crisp, and cheese is fully melted, about 3 to 4 minutes per side.
05 -
Transfer sandwiches to a cutting board, slice if desired, and serve immediately with warm au jus on the side for dipping.