If you haven't already, cook your chicken breasts by boiling them in water for 20 minutes, or until juices run clear when cut into. Then cube and set aside.

Preheat the oven to 350. Chop the onion into small pieces and saute in 1/4 cup butter over medium heat for 5 minutes or until slightly translucent. Turn heat down to low and add both cans of soup, stirring until mixed well. Remove from heat and set aside.

In a prepared 9 x 13 baking dish place a layer of tortillas to cover the bottom of the dish. Add a thin layer of tomatoes and green chilis, a thin layer of the soup mixture, a thin layer of chicken, and top with cheese. Add another thin layer of tortillas, tomatoes and green chilis, soup mixture, chicken, and cheese. Keep doing the layers until you run out of space, finally covering the dish with the remaining cheese.

Bake in the preheated oven on 350 for 45 minutes, or until cheese is turned a golden brown. Allow to cool before serving.