Heat a large cast iron skillet or heavy-bottomed pan over medium-high heat. Add the ground beef and allow it to sit undisturbed for a few minutes so it develops a deep brown crust—a key step in building flavor. Once the bottom has browned, break the meat into small pieces and continue cooking until about half the beef has a rich, golden crust. Remove the cooked beef from the skillet, leaving any rendered fat behind.

Add the butter to the skillet. Stir in the diced onions, bell peppers, and mushrooms, spreading them out in an even layer. Let the vegetables cook without stirring for 1–2 minutes to encourage caramelization, then stir and cook for another 1–2 minutes. This browning step adds a deep, naturally sweet flavor that makes the filling more complex and satisfying.

Return the cooked beef to the skillet. In a small bowl, whisk together the beef broth and cornstarch until no lumps remain. Pour this mixture into the pan along with the ketchup, Worcestershire sauce, salt, and black pepper. Stir everything together, then let the mixture simmer for 3–5 minutes. As it cooks, the sauce will thicken and cling to the meat and vegetables, giving you the hearty, spoonable filling that defines a great sloppy joe.

When the mixture has thickened, turn off the heat and add the chopped provolone. Stir until the cheese melts completely and coats the filling in a velvety, cheesy layer. This is where the philly cheesesteak flavor truly comes alive.

Toast the brioche buns for a few minutes to give them structure and a touch of crispness. Then spoon generous amounts of the warm, cheesy filling onto each bun. Serve immediately and enjoy a comforting, savory philly cheesesteak–style dinner that’s easy enough for weeknights but delicious enough to share with guests.