Heat oven to 350°F. Cook and drain spaghetti as directed on package, adding peas during last 3 minutes of cooking.
Meanwhile, in 3-quart saucepan, melt butter over low heat. Stir in flour, salt and pepper. Cook, stirring constantly, until mixture is smooth and bubbly; remove from heat.
Stir in broth and whipping cream. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in sherry, spaghetti and peas, chicken and mushrooms.
Into ungreased 2-quart casserole, pour spaghetti mixture. Sprinkle with cheese. Bake uncovered about 30 minutes or until bubbly in center.