Heat the Skillet: Preheat a large cast-iron skillet over medium-high heat.
Cook the Beef: Add ground beef and cook without stirring until browned. Break it apart and cook fully. Remove from skillet, leaving any fat behind.
Sauté Vegetables: Add butter to the skillet. Sauté onion, bell pepper, and mushrooms for 4–5 minutes until softened and caramelized.
Combine Beef and Veggies: Return the beef to the skillet, stirring to combine with the vegetables.
Prepare the Sauce: In a small bowl, mix beef broth and cornstarch. Pour into the skillet along with ketchup, Worcestershire sauce, salt, and black pepper. Stir well and simmer for 3–5 minutes until the mixture thickens slightly.
Add Cheese: Turn off the heat. Stir in chopped provolone cheese until fully melted and gooey.
Toast the Buns: Lightly toast brioche buns for extra flavor and sturdiness.
Assemble & Serve: Spoon the cheesy beef mixture onto the toasted buns. Serve warm and enjoy!