1. In a large skillet, heat the butter and olive oil over medium heat. Add the sliced onions and sauté for about 10-15 minutes, stirring occasionally, until they are caramelized and golden brown.
2. Add the ground beef to the skillet with the onions. Cook until the beef is browned and cooked through, breaking it apart with a spatula as it cooks.
3. Stir in the garlic powder, Worcestershire sauce, and beef broth. Season with salt and pepper to taste. Let the mixture simmer for about 5-7 minutes, allowing the flavors to meld and the sauce to thicken slightly.
4. While the beef mixture is simmering, preheat your oven to 350°F (175°C).
5. Once the beef mixture is ready, spoon it generously onto the bottom halves of the hamburger buns. Top each with a generous amount of shredded Swiss cheese.
6. Place the assembled buns on a baking sheet and bake in the preheated oven for about 5-7 minutes, or until the cheese is melted and bubbly.
7. Remove from the oven, garnish with fresh parsley, and serve immediately with the top halves of the buns.
Servings: 4
Prep Time: 10 minutes
Cook Time: 25 minutes