Move oven rack to the middle position, and preheat to 350 degrees F. Lightly spray an 8-inch square baking dish with nonstick cooking spray; set aside.
In a large skillet over medium heat add onion, pepper, and Italian dressing. Cook, stirring frequently until onions become translucent and soften about 5 minutes.
Add cream cheese to the pan and use back of a wooden spoon to break it into chunks.
Once cream cheese starts to melt add Greek yogurt and American cheese. Stir constantly until all the cheese melts. Turn heat off and stir in chopped roast beef. Season with salt and pepper.
Pour mixture into prepared baking dish. Bake for 15 minutes, sprinkle with Parmesan cheese, then bake for 10 more minutes, or until the edges are bubbly. Serve with sliced French bread and tortilla chips.
*The nutritional information provided is just for the Philly Cheesesteak dip, not for the bread or chips used for dipping.