Preheat the oven to 375 degrees and spray a large loaf pan with cooking spray.

In a large skillet, add the butter and the onions and bell peppers, mushrooms, salt and pepper.

Let brown for 1-2 minutes before stirring, then let brown for another 1-2 minutes before stirring again.

Let cool for 5 minutes.

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

Add half the mixture to your loaf pan then add half of the cheese to the middle of the pan in a 2 inch wide section.

Cover with the rest of the meat and form into a rounded top loaf shape.

Cook for 40 minutes, then pull out of the oven, cover with remaining cheese and put back in.

Cook for 15-20 minutes, then let rest for 10 minutes before cutting.