Sear the Ground Beef: Heat a large skillet over medium-high heat, crumble the ground beef into the pan, sear without stirring for about 1 minute until browned on the bottom.

Break Up the Meat: Use a spatula to break the browned beef into smaller pieces, cook for an additional 2-3 minutes until no pink remains. Set aside in a bowl.

Sauté the Vegetables: Add butter to the skillet, melt, then add diced onions, green bell peppers, and minced mushrooms. Sauté for 5-7 minutes until softened and golden.

Prepare the Sauce: In a bowl, combine beef broth, cornstarch, ketchup, Worcestershire sauce, kosher salt, and black pepper. Whisk until well-mixed.

Combine Sauce with Vegetables: Pour the sauce into the skillet over the sautéed vegetables. Stir to coat, increase heat slightly and bring to a bubble. Simmer for 3-5 minutes until thickened.

Mix in the Beef: Return the cooked beef to the skillet, stir into the vegetable and sauce mixture. Cook for an additional 2-3 minutes on medium heat.

Melt the Cheese: Remove from heat and add shredded provolone cheese to the beef mixture. Fold gently until cheese is melted and creamy.

Toast the Buns: Butter buns on both sides, heat a separate skillet over medium heat, toast buns until golden brown, about 2-3 minutes.

Assemble and Serve: Fill each toasted brioche bun with the cheesy beef mixture and serve immediately.