Combine chicken, onion, 1 teaspoon of the salt, peppercorns and enough water to cover in a kettle.
Heat to boiling; cover.
Cook 40 minutes, or until chicken is tender.
Remove from broth and cool until easy to handle.
Strain broth into a 4 cup measure and set aside for making sauce.
Pull skin from chicken and take meat from bones; cube meat; place in a large bowl.
Saute mushrooms in 1/4 cup of the butter or margarine until soft in a large frying pan; combine with chicken.
Melt remaining butter or margarine in a saucepan.
Measure out 1/4 cup and toss with bread crumbs in a bowl; set aside.
Stir flour, the remaining 1 1/2 teaspoons salt, pepper and nutmeg into remaining butter or margarine in saucepan; cook, stirring constantly, until bubbly.
Stir in 3 1/2 cups of the chicken broth and cream.
Continue cooking and stirring until sauce thickens and boils 1 minute; remove from heat Stir in sherry.
While sauce cooks, cook spaghetti, following label directions; drain well.
Spoon into two baking dishes, 13x9x2.
Spoon chicken mixture over spaghetti; spoon sauce over all.
Add Parmesan cheese to bread crumb mixture and toss lightly to mix.
Sprinkle over mixture in baking dishes.
Bake in moderate oven (350°) 30 minutes, or until bubbly and crumb topping is toasted.
Garnish with bouquets of watercress, sliced mushrooms and pimiento strips, if you wish.
DAY BEFORE NOTE:
Fix the 2 casseroles through Step 7; cover and chill.
About an hour before serving, remove from refrigerator and uncover.
Bake in moderate oven (350°) 40 minutes, or until bubbly.
If casseroles must stand a bit before serving, leave in oven with heat turned off.