In a large skillet over medium heat, add butter, onion, bell pepper, and mushrooms. Sauté for a few minutes until veggies start to soften.
Add ground beef. Cook until no longer pink and veggies are soft.
In a bowl, mix beef broth and cornstarch.
Add ketchup, Worcestershire sauce, salt, pepper, and the beef broth/cornstarch mixture to the pan.
Cook for ~5 minutes until simmering and well combined.
Layer provolone cheese on top of the beef mixture and let melt.
Scoop mixture onto brioche buns. Serve and enjoy!
