Preheat the oven to 375°F and lightly coat a large loaf pan with cooking spray. In a large skillet, melt the butter and add the diced onions, bell pepper, and mushrooms. Season with salt and black pepper. Sauté the mixture, allowing it to brown undisturbed for 1 to 2 minutes before stirring, then repeat the browning process once more. Remove from heat and let cool for five minutes.

In a spacious mixing bowl, combine the ground beef, ketchup, Worcestershire sauce, eggs, panko breadcrumbs, and the cooled sautéed vegetables. Mix until well blended.

Transfer half of the meat mixture into the prepared loaf pan and press it evenly. Arrange half of the Provolone cheese slices (cut into thirds to fit) in a 2-inch wide strip along the center. Layer the remaining meat mixture over the top, shaping it into a domed loaf.

Place the loaf in the oven and bake for 40 minutes. Remove it briefly, cover with the remaining Provolone slices, and return to the oven for an additional 15 to 20 minutes.

Allow the meatloaf to rest for 10 minutes prior to slicing and serving.