Preheat the oven to 350°F. Grease a 9x5 inch loaf pan.

In a large bowl, combine ground beef, breadcrumbs, eggs, milk, diced green bell pepper, diced onion, garlic powder, salt, and black pepper.

Mix until just combined.

Heat olive oil in a skillet over medium heat. Add sliced onion, sliced green bell pepper, and mushrooms.

Cook until vegetables are softened, about 5-7 minutes. Stir in Worcestershire sauce, and season with salt and pepper.

Remove from heat and let cool slightly.

Press half of the meatloaf mixture into the bottom of the prepared loaf pan. Spread the cooked vegetable mixture over the meat.

Lay 6 slices of provolone cheese over the vegetable mixture, covering it completely.

Top with the remaining meatloaf mixture, pressing gently to seal the edges around the cheese.

Place the loaf pan in the preheated oven and bake for 45 minutes.

Remove meatloaf from the oven and sprinkle evenly with shredded provolone and mozzarella cheese.

Return to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.

Let the meatloaf rest for 10 minutes before slicing. Serve warm.