Heat butter in a large skillet over medium heat.
Add the sliced onions and sauté until golden brown, about 15 minutes.
Sprinkle in the garlic powder, thyme, salt, and pepper; stir well.
Increase the heat to medium-high and add the ground beef, breaking it apart as it cooks.
Pour in the Worcestershire sauce and beef broth; stir to combine.
Let simmer for about 10 minutes until the mixture thickens slightly, stirring occasionally.
Toast the hamburger buns lightly on a separate skillet if desired.
Spoon the beef mixture onto the bottom half of each bun, then top with shredded Gruyère cheese.
Place the top bun on and serve immediately.