1. Heat the butter and olive oil in a large skillet over medium heat. Add the sliced onions and cook for 12 to 15 minutes, stirring often, until soft and lightly golden.

2. Add the ground beef to the skillet and cook until browned, breaking it up with a spoon as it cooks. Drain excess grease if needed.

3. Stir in the minced garlic, onion soup mix, black pepper, and salt. Cook for 1 minute until fragrant.

4. Add the beef broth, Worcestershire sauce, ketchup, and Dijon mustard. Stir well and simmer for 5 to 7 minutes until the mixture thickens slightly.

5. Toast the buns lightly in the oven or in a dry skillet.

6. Spoon the beef mixture onto the bottom buns and top with mozzarella or provolone cheese.

7. Broil briefly if desired until the cheese is melted and bubbly.

8. Add the top buns, sprinkle with parsley if using, and serve hot.