01 -
Melt the butter in a large skillet over medium-low heat. Add the sliced onions and thyme, stirring occasionally. Cook slowly for 20 to 25 minutes until the onions are deeply golden and jam-like in texture.
02 -
Push the caramelized onions to one side of the skillet and add the ground beef. Cook over medium heat, breaking the meat apart, until fully browned. Combine the beef and onions thoroughly.
03 -
Add the minced garlic to the skillet and cook for 1 minute. Pour in the beef broth and Worcestershire sauce. Simmer the mixture for 5 to 10 minutes until thickened and rich in flavor.
04 -
While the filling simmers, toast the brioche buns in a dry pan or oven until golden and crisp to prevent sogginess.
05 -
Place a generous portion of the beef and onion mixture on the bottom half of each toasted bun. Top with a slice of provolone or Gruyère cheese. Cover the skillet briefly to melt the cheese before closing the sandwich with the top bun.