Preheat oven to 375°F (190°C).
In a large bowl, mix together ground beef, steak, onions, green bell pepper, garlic, bread crumbs, milk, egg, Worcestershire sauce, salt, and black pepper until well combined.
Press half of the meat mixture into a loaf pan.
Sprinkle the shredded provolone cheese over the meat in the pan, then top with the remaining meat mixture, pressing down gently.
In a small bowl, mix together ketchup, brown sugar, and mustard. Spread this mixture over the top of the meatloaf.
Bake for 50-60 minutes, or until the meatloaf is cooked through.
Let the meatloaf rest for 10 minutes before slicing and serving.