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 5–7 minutes. Drain any excess fat if necessary.
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 evenly coat the beef and vegetables.
04 - Pour in the beef broth and bring to a gentle simmer. Cook for 5–7 minutes, stirring occasionally, until the mixture thickens slightly and the flavors meld together.
05 - Reduce heat to low. Sprinkle the shredded provolone cheese evenly over the beef mixture. Cover the pan and let the cheese melt for about 2 minutes until fully melted and gooey.
06 - Stir the cheesy beef mixture together 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.