Begin by preheating your oven to 375°F. Prepare a large (9x5) loaf pan by spraying it with non-stick cooking spray.

In a large skillet, melt the butter over medium heat. Add the chopped onions, bell peppers, and mushrooms, seasoning with salt and pepper. Cook for about 1-2 minutes, then continue to cook until browned, approximately 1-2 more minutes. Remove from heat and let cool for 5 minutes.

In a large mixing bowl, combine ketchup, Worcestershire sauce, and eggs, whisking until smooth. Add the ground beef, sautéed vegetables, and panko breadcrumbs. Mix everything together thoroughly with your hands until well blended.

Press half of the meat mixture into the prepared loaf pan. Arrange half of the Provolone cheese slices in a single layer down the center of the meatloaf, about 2 inches wide. Cover with the remaining meat mixture.

Bake the meatloaf for 40 minutes. Remove from the oven, place the remaining Provolone cheese slices on top, and return to the oven. Bake for an additional 15-20 minutes, until the cheese is melted and bubbly.

Allow the meatloaf to rest for 10 minutes before slicing and serving. Enjoy!