Instructions:
Heat oil in a large pan over medium heat. Add diced onion, bell pepper, and mushrooms. Cook until tender, about 7-10 minutes.
Add chopped garlic and cook until fragrant (about 1 minute) before removing from heat.
In a mixing bowl, combine ground beef, salt, pepper, cheese whiz (if using), Worcestershire sauce, ketchup, eggs, breadcrumbs, and the cooked onion, pepper, mushroom, and garlic mixture.
Place half of the meat mixture on a baking sheet. Add a solid piece of provolone in the middle. Top with the remaining meat mixture, seal the edges well, shape the loaf as desired.
Bake in a preheated 350F/180C oven until cooked through, about 50-60 minutes.
Spread sliced provolone over the top of the loaf, return to the oven, and broil until the cheese has melted, about 2-4 minutes.