Brown ground beef and onion; drain off grease. Set aside. Mix mushroomsoup and tomatoes in blender. Add to ground beef. In a greased 9x13 inchcasserole dish, layer 5 to 6 tortillas, torn into bite size pieces, halfthe ground beef mixture and half the grated cheese. Repeat layers; endingwith cheese. Bake at 180ºC or 30 minutes.