1. Heat the butter and olive oil in a large skillet over medium heat. Add the sliced onions and salt, then cook for 15 to 20 minutes, stirring often, until soft and golden brown.

2. Add the minced garlic and cook for 30 seconds until fragrant.

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

4. Stir in the beef broth, Worcestershire sauce, onion soup mix, and black pepper. Mix well.

5. In a small bowl, whisk together the cornstarch and water until smooth. Stir it into the skillet and simmer for 2 to 3 minutes until the mixture thickens.

6. Lightly toast the buns if desired. Spoon the beef mixture onto the bottom halves of the buns.

7. Top each sandwich with provolone cheese, sprinkle with parsley, add the top buns, and serve warm.