Brown ground beef and onion; drain off grease. Set aside. Mix mushroom soup and tomatoes in blender. Add to ground beef. In a greased 9x13 inch casserole dish, layer 5 to 6 tortillas, torn into bite size pieces, half the ground beef mixture and 1/2 the grated cheese. Repeat layers; ending with cheese. Bake at 35 degrees for 3 minutes.