Cook hen in water until tender. Remove and debone. Skim fat from broth to use in sauce. Combine fat, tomatoes, celery, bell pepper, onion, mushrooms and garlic. Simmer abt. 3 hrs., adding broth as needed. Cook spaghetti in remaining broth, adding a little water if necessary. Add a couple of bouillon cubes to boost flavor, if desired. Drain. In 3 to 5 litre casserole dish layer spaghetti, chicken, sauce and cheese. Put last layer of cheese on just before serving.