Place chicken in hot water and add celery salt and onion salt. Simmer until fork tender, 1 - 1/2 hours.
Remove chicken and place in large bowl to cool. When able to handle remove pieces from bone, keeping as large as possible. Cut breasts into thirds, refrigerate.
In the meantime remove 2 cups of broth from pan and set aside. Then add 2 quarts so you have 12 cups in all of water to remainder of broth in pan. Add a little more salt and bring to boil. Place spagettini in a little at a time so as water will keep boiling. Cook for about 6 minutes or until tender crisp.
In the meantime place 3 tbsp of butter in a skillet and melt. Then add cut up mushrooms. sprinkle with lemon juice and salt. Saute until tender not brown. While mushrooms are cooking drain spagettini and place in a 8 X 12 X 8 casserole dish. Toss mushrooms and their butter with the spagettini. Cover and place in Refrigerator.
Now melt 3 tbsp of butter in a sauce pan. Remove from fire. combine 2 tbsp of flour, pepper, salt, paprika and nutmeg in a cup and then add to butter a little at a time, mixing completely. Then slowly add the 2 cups of reserved broth into this mixture. Then place on a high flame stirring constantly. When this is thickened remove from fire and stir in one cup of heavy cream. Cover this chicken cream sauce and refrigerate.
The next day one hour before serving, preheat oven to 400º Remove casserole from refrigerator. Pour the chicken sauce over the spagettini and then place the boned chicken pieces on top. Sprinkle with the Parmesan cheese and paprika. Bake for 25 - 30 minutes until bubbly hot.