To reheat Philly cheesesteak, you can follow these simple steps: Preheat your oven to 350°F (180°C). Take the leftover vegetarian cheese steak out of the refrigerator. Wrap the vegetarian cheese steak mushroom slices in aluminum foil, making sure it's sealed tightly. Place the foil-wrapped cheesesteak on a baking sheet and put it in the oven. Heat for 10-15 minutes or until the cheese is melted and the mushroom slices are heated through. Alternatively, you can reheat it on a stovetop by heating a pan over medium-high heat and placing the vegetarian cheese steak slices in the pan for about 2-3 minutes on each side until it's heated all the way through. Enjoy!