1. Preheat your oven to 350°F (175°C) and lightly grease a 9×5 inch loaf pan or line a baking sheet with parchment paper.
2. Heat 2 tablespoons of olive oil in a large skillet over medium heat.
3. Add the chopped onions and cook for 2-3 minutes until they begin to soften.
4. Add the diced green bell peppers and sliced mushrooms to the skillet, and sauté for another 5-7 minutes until the vegetables are tender and lightly caramelized.
5. Stir in the minced garlic and cook for an additional 30 seconds until fragrant.
6. Remove the skillet from heat and allow the vegetable mixture to cool completely.
7. In a large mixing bowl, combine ground beef, cooled sautéed vegetables, panko breadcrumbs, beaten egg, milk, ketchup, Worcestershire sauce, dried oregano, salt, and black pepper.
8. Using clean hands or a large fork, gently mix the ingredients until just combined. Avoid overmixing to keep the meatloaf tender.
9. Divide the meat mixture in half. Press half of the mixture into the bottom of the prepared loaf pan or shape it into a rectangular loaf on the baking sheet.
10. Create a shallow well in the center of the meat and arrange cubed provolone cheese along the length of the loaf, leaving a border around the edges.
11. Cover the cheese layer with the remaining meat mixture, carefully sealing the edges to enclose the cheese completely.
12. Shape the meatloaf into an even rectangle if using a baking sheet and ensure no gaps remain.
13. Spread a thin layer of ketchup over the top of the meatloaf for added flavor and moisture.
14. Bake the meatloaf in the preheated oven for 45-55 minutes or until the internal temperature reaches 160°F (71°C).
15. Approximately 5 minutes before the meatloaf is done, place sliced provolone cheese on top of the loaf and let it melt in the oven.
16. Remove the meatloaf from the oven and let it rest for 5-10 minutes before slicing to allow the juices to redistribute.
17. Slice and serve the Philly Cheesesteak Meatloaf with your favorite sides or a fresh salad.