In a large skillet over medium heat, add a drizzle of oil and sauté the diced onions, green peppers, and sliced mushrooms until they become tender and slightly golden, about 5-7 minutes. Stir occasionally to prevent sticking, and then remove from heat. Allow the sautéed vegetables to cool for a few minutes while you prepare the meat mixture.

In a spacious mixing bowl, combine the ground beef, breadcrumbs, egg, milk, Worcestershire sauce, garlic powder, and the cooled sautéed vegetables. Season with salt and pepper to taste. Gently mix the ingredients using your hands or a spatula, being careful not to overwork the mixture.

Preheat your oven to 350°F (175°C) and grease a loaf pan. In the prepared pan, press half of the meat mixture evenly across the bottom. Layer slices of provolone cheese on top, then top with the remaining meat mixture, sealing the edges well.

In a small bowl, whisk together the beef broth, ketchup, brown sugar, and mustard. Brush half of the glaze over the top of the meatloaf for a delicious coating.

Place your assembled meatloaf in the preheated oven and bake for 45-50 minutes, or until the internal temperature reaches 160°F (70°C). During the last 10 minutes of baking, brush on the remaining glaze.

Once baked, remove the meatloaf from the oven and let it rest for 10 minutes. Slice it carefully, garnish with fresh parsley if desired, and serve with your favorite sides.