01 - Heat a large nonstick skillet over medium-high heat. Add the ground beef and cook, breaking it apart with a wooden spoon, until thoroughly browned, about 5 to 7 minutes. Drain any excess fat if needed.
02 - Add the diced onion, bell pepper, mushrooms (if using), and minced garlic to the skillet. Sauté until the vegetables are softened, about 5 minutes.
03 - Stir in the Worcestershire sauce, ketchup, Dijon mustard, smoked paprika, kosher salt, black pepper, and chili flakes. Mix thoroughly to evenly coat the beef and vegetables.
04 - Pour in the beef broth and bring to a gentle simmer. Cook for 5 to 7 minutes, allowing the mixture to reduce and thicken slightly.
05 - Reduce heat to low. Sprinkle the shredded provolone cheese evenly over the beef mixture. Cover the skillet and let stand until the cheese melts, about 2 minutes.
06 - Stir the melted cheese into the beef mixture until well combined and creamy throughout.
07 - Spoon generous portions of the cheesy beef filling onto the toasted buns. Garnish with chopped parsley and sliced jalapeños if desired. Serve immediately while hot.