Heat butter in large skillet over medium heat.
Add sliced onions to the hot butter carefully.
Cook onions until golden brown and caramelized completely.
This takes about 8-10 minutes of cooking time.
Add ground beef to the caramelized onions.
Break meat apart with wooden spoon thoroughly.
Cook until beef is browned on all sides.
Drain excess fat from the skillet if needed.
Sprinkle French onion soup mix over beef mixture.
Stir ingredients together until evenly coated and mixed.
Pour beef broth into the skillet slowly.
Add Worcestershire sauce and brown sugar to mixture.
Simmer everything together for 5-7 minutes until thickened.
Season with salt and pepper to taste preferences.
Toast hamburger buns until lightly golden brown outside.
Spoon beef mixture onto bottom bun halves generously.
Top each serving with one Swiss cheese slice.
Cover with top bun halves and serve immediately.