Preheat oven to 350 degrees Fahrenheit. Combine sauce, tomatoes, chicken and onion in saucepan. Stir and cook over medium heat until bubbly. Layer 3 tortillas flat on bottom of 2-quart casserole dish, covering bottom. Pour 1/3 of chicken/sauce mixture over tortillas and cover with 1/3 of grated cheese. Repeat twice more, ending with a thick layer cheese on top. Bake in preheated oven for 20 min. Allow to sit for at least 10 minutes before serving. Serves 6.