A perennial comfort food. And, by its nature, it begs for tinkering to the ingredients on hand, including seasonings and (even more often than chicken) a post-holiday turkey carcass. 

Ingredients:

1½ Chickens (fryers)
Water (to cover in the pot)
1 tsp. Onion Salt
½ tsp. Celery Salt

½ lb. Vermicelli (or similar spaghetti-like pasta)
Salt 

6 tbsp. Butter
¾ lb. Mushrooms, sliced
1 tbsp. Lemon Juice

¼ tsp. Paprika
¼ tsp. Pepper
1/8 tsp. Nutmeg
1 c. Milk
1/3 c. Parmesan Cheese

Instructions:

Chicken: 

1. Place chicken, salts, water in large pan. Simmer covered 30 min, until tender. 
2. Remove chicken, save broth.
3. Cool chicken, remove meat from bones, refrigerate.

Pasta:

1. Reserve 2½ cups of broth from the previous step.
2. Add water to remaining broth to fill the pan. Add 2 tbsp. salt.
3. Bring to boil, add pasta.Cook 6 minutes.
4. Drain, return to pan or large bowl.

Mushrooms:

1. In frying pan heat 3 tbsp. butter.
2. Add mushrooms, sprinkle with lemon juice, ½ tbsp. salt.
3. Sauté until soft but not brown.
4. Put with pasta.

Sauce:

1. Melt 3 tbsp. butter in pan.
2. Stir in flour, paprika, 1½ tsp. salt, pepper, nutmeg. Cook 5 minutes.
3. Slowly stir in reserved broth. Cook until it boils (it will be thin).
4. Add milk.
5. Mix everything together in a large bowl. Place in a 9x13 baking dish. Sprinkle generously with parmesan cheese.
6. Bake covered at 400F for 20 minutes. Uncover and add more cheese before cooking another 10 minutes.

Serves: 8-10 if half are ladies. On the other hand, a family of four can polish it off without much trouble.

Prep time: 1:30

Notes:

• This will keep two days refrigerated. Oven heat from the fridge 45 minutes to an hour, until bubbly. Freezes well. As with most cassaroles, many folks consider "aging" in the fridge or freezer an improvement over "fresh."
• If you are dealing with an existing carcass (chicken or turkey), pick the (presumably cooked) carcass from the bones. Use the (meaty) bones in water with with some celary, onions, maybe garlic salt and pepper, to simmer as described to make the broth. The amount of meat you pick off first will determine proportions on the recipe. You can also substitute chicken tenders or boneless breasts.
• Yes, it's spelled "Tetrazzini," not "Tetrazini."

Source: Ann Coombs, as tweaked on more than one occasion by Margie.