Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.

In a skillet, melt butter over medium heat. Add onion, bell pepper, and mushrooms. Sauté for 5–7 minutes until soft. Season with salt and pepper. Let cool.

In a large mixing bowl, combine the eggs, ketchup, Worcestershire sauce, ground beef, panko breadcrumbs, and sautéed veggies. Mix gently until fully combined.

Divide the meat mixture in half. Shape the first half into a loaf on the prepared baking sheet.

Layer half the provolone slices over the loaf, leaving a small border.

Cover with the remaining meat mixture and shape it to seal in the cheese.

Bake for 45–50 minutes, or until the internal temperature reaches 160°F (71°C).

In the last 5 minutes of baking, top with remaining provolone slices and let melt.

Let rest for 5–10 minutes before slicing. Serve warm.