Preheat your oven to 350°F (175°C).
In a large bowl, combine ground beef, bread crumbs, egg, milk, onion, green pepper, mushrooms, Worcestershire sauce, garlic powder, salt, and pepper. Mix well.
Press half of the meat mixture into a loaf pan. Lay provolone slices over the meat, leaving a border.
Cover with the remaining meat mixture, pressing to seal edges.
In a small bowl, whisk beef broth, ketchup, brown sugar, and mustard. Pour the sauce evenly over the meatloaf.
Bake for 1 hour, or until the top is golden brown and the meatloaf is fully cooked.
Let the meatloaf rest for 10 minutes before slicing. Serve warm.