Preheat your oven to 375°F (190°C) and spray a large loaf pan with cooking spray.

In a large skillet, melt the butter over medium heat, then add the diced onions, green bell peppers, and minced mushrooms. Sprinkle with Kosher salt and black pepper.

Let the vegetables brown for 1–2 minutes without stirring, then stir and let them brown for another 1–2 minutes. Allow the mixture to cool for five minutes.

In a large bowl, combine the lean ground beef, ketchup, Worcestershire sauce, eggs, panko breadcrumbs, and the cooled vegetable mixture.

Mix it together until well combined using your hands or a spoon.

Spread half of the meat mixture into the prepared loaf pan.

Place half of the Provolone cheese slices in the center, forming a 2-inch wide section.

Cover the cheese with the remaining meat mixture, shaping it into a rounded loaf.

Bake for 40 minutes, then add the remaining cheese slices on top and bake for an additional 15–20 minutes.

Allow the meatloaf to rest for 10 minutes before slicing.