01 - Heat a large nonstick skillet over medium-high heat. Add the ground beef and cook, breaking it apart with a wooden spoon, until evenly browned throughout, 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 and fragrant, about 5 minutes.
03 - Stir in the Worcestershire sauce, ketchup, Dijon mustard, smoked paprika, kosher salt, black pepper, and chili flakes. Mix thoroughly to coat the beef and vegetables evenly.
04 - Pour in the beef broth and bring to a gentle simmer. Cook for 5 to 7 minutes, allowing the liquid to reduce and the mixture to thicken slightly.
05 - Reduce the heat to low. Sprinkle the shredded provolone cheese evenly over the beef mixture. Cover the skillet and let stand until the cheese is fully melted, about 2 minutes.
06 - Stir the cheesy beef mixture together until creamy and well combined. Spoon generous portions onto the toasted buns. Garnish with chopped parsley or sliced jalapeños if desired. Serve immediately while hot.