Brown the meat and onion together, drain. In a saucepan, cook the soup, chilies, crushed tomatoes, and tomato sauce. Add the browned meat and simmer on low heat. In a frying pan heat a little vegetable oil and heat the corn tortillas until they are warm but still soft. Place 4 of the warmed tortillas in the base of a casserole dish and top with 1/2 of the meat mixture. Sprinkle with 1/2 of the cheese. Repeat the layers, starting with the final 2 tortillas, meat mixture and remaining cheese. Bake for 30 minutes.