Preheat the oven to 375°F (190°C). Grease a loaf pan or line with parchment paper. In a skillet over medium heat, sauté the onion, bell pepper, mushrooms, and garlic until softened. Remove from heat and let cool slightly.

In a large bowl, combine ground beef, sautéed vegetables, Worcestershire sauce, salt, pepper, Italian seasoning, shredded cheese, breadcrumbs, egg, and milk. Mix until just combined.

Transfer the mixture to the loaf pan and shape into an even loaf. Bake for 40-45 minutes or until the internal temperature reaches 160°F (71°C).

Remove from the oven, place provolone slices on top, and bake for another 5 minutes until the cheese melts. Let rest for 5 minutes before slicing.

Serve topped with additional sautéed onions and bell peppers if desired.