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 off 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 the mixture to a gentle simmer. Cook for 5 to 7 minutes, stirring occasionally, until the liquid reduces and the mixture thickens slightly.
05 - Reduce the heat to low. Sprinkle the shredded provolone cheese evenly over the beef mixture. Cover the skillet and let the cheese melt completely, about 2 minutes.
06 - Stir the melted cheese into the beef mixture until well combined and creamy. Spoon generous portions onto the toasted buns. Garnish with chopped parsley or sliced jalapeños if desired. Serve immediately while hot.