01 -
Melt butter in a large skillet over medium-low heat, add sliced onions and thyme. Stir occasionally and cook for 20-25 minutes until onions are golden brown and jammy.
02 -
Push the onions to the side of the skillet, add ground beef and cook until fully browned, breaking it apart with a wooden spoon. Combine with the caramelized onions.
03 -
Add minced garlic and cook for 1 minute. Pour in beef broth and Worcestershire sauce, then simmer for 5-10 minutes until the mixture thickens and flavors meld.
04 -
While the mixture simmers, toast the brioche buns in a toaster or oven until golden and slightly crisp.
05 -
Place a generous portion of the beef and onion mixture on the bottom half of each bun, top with a slice of cheese, cover briefly to melt the cheese, then close the sandwich.