Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium-high heat.
Add the thinly sliced beef and cook until it loses its pink color.
Season the beef with salt and black pepper as it cooks. Remove the cooked beef from the skillet and set aside.
In the same skillet, add the sliced onions, green peppers, and mushrooms. Sauté until tender and lightly caramelized, about 5 to 7 minutes. Set aside.
In a medium bowl, combine the cream cheese and mayonnaise until smooth. Stir in the grated Parmesan cheese and half of the grated provolone and mozzarella cheeses. Mix until creamy.
In a small bowl, combine the garlic powder, onion powder, dried oregano, dried basil, red pepper flakes, salt, and black pepper. This is your seasoning mix.
Arrange half of the cooked beef in the bottom of a greased 9 x 13-inch baking dish. Sprinkle half of the seasoning mix over the beef.
Arrange the sautéed onions, peppers, and mushrooms over the beef.
Spread the creamy cheese mixture over the vegetables.
Arrange the remaining beef over the cheese mixture and sprinkle with the remaining seasoning mix.
Finally, sprinkle with the remaining shredded provolone and mozzarella cheese.
Bake in the preheated oven for 25 to 30 minutes, or until the casserole is hot and bubbly, and the cheese is melted and golden.
Remove the casserole from the oven and let it rest for a few minutes before serving. Serve your Philly Cheesesteak Gratin warm and enjoy!
This gratin combines all the flavors of a classic Philly Cheesesteak in a convenient and delicious dish.
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