Preheat oven to 350°.
Melt butter in a saucepan over medium heat. Add mushrooms and shallots; sauté 5 minutes. Stir in flour. Gradually add milk, broth, and sherry; stir with a whisk until blended. Bring to a boil; reduce heat. Simmer 8 minutes; stir constantly. Stir in chicken and 1/2 cup cheese; cook 1 minute. Remove from heat; add pasta, nuts, salt, pepper, and nutmeg. Spoon into a 3-quart casserole coated with cooking spray; top with 2 tablespoons cheese. Bake at 350° for 40 minutes. Let stand 5 minutes.