Peel and cut the potatoes into 1/4-inch sticks. Place into a medium bowl and mix with the remaining ingredients. Bake at 400F for 25-30 minutes.
Meanwhile, heat up a large skillet on medium-high heat. Add olive oil, bells peppers and mushrooms and cook for about 7 minutes. Season with salt and pepper and remove the veggies from the skillet.
To the same skillet, add shaved steak. Cook for 5-7 minutes, until it turns brown and no pink color remains. Season with salt, pepper and red pepper flakes. Stir, add the cheese singles and the cooked veggies. Turn off the heat and give the mixture a final stir.
Remove the fries from the oven and arrange onto serving plate(s). Add the Philly Cheese Steak mixture on top. Garnish with chopped parsley if desired. Serve immediately.