Preheat the oven to 375°F. Lightly grease a large loaf pan with cooking spray to prevent sticking.

Melt the butter in a large skillet over medium heat. Add the diced onion, bell pepper, and minced mushrooms. Season with salt and pepper. Cook undisturbed for 1-2 minutes to achieve light browning, then stir. Repeat the process until the vegetables are tender and golden. Remove from heat and allow the mixture to cool for 5 minutes.

In a large mixing bowl, combine the ground beef, panko breadcrumbs, eggs, ketchup, Worcestershire sauce, and the cooled vegetable mixture. Mix gently until all ingredients are evenly incorporated.

Spread half of the meat mixture evenly into the prepared loaf pan. Arrange half of the Provolone cheese slices in a 2-inch-wide strip along the center. Ensure the cheese stays within the middle to prevent leakage.

Cover the cheese layer with the remaining meat mixture, shaping it into a domed loaf for even cooking.

Transfer the pan to the preheated oven and bake for 40 minutes. Remove from the oven and layer the remaining Provolone cheese on top. Return the pan to the oven and bake for an additional 15-20 minutes, or until the cheese is melted and the meatloaf reaches an internal temperature of 160°F.

Once baked, allow the meatloaf to rest for 10 minutes to set. Carefully slice and serve warm.