Preheat oven to 350°.
Salt and pepper the chicken breasts and grill over medium heat 8-10 minutes a side. Let rest for 5 minutes, then using two forks, shred the chicken breasts and set aside.
Combine soup, tomatoes, and Fajita seasoning in a bowl and mix well.
Spray a 13" x 9" pan with cooking spray. Layer four of the ripped tortillas on the bottom of the pan.
Layer a third of the shredded chicken, a third of the grated cheese, and a third of the soup mixture.
Repeat two more layers. Add two more torn tortillas on top with some more of the grated cheese.
Bake at 350° for one hour. Let rest for 5 minutes before serving.